Intermittent fasting can last anywhere from about 12 hours to a full day or more, depending on the method. For beginners, 12:12 or 14:10 is usually a gentler start, while 16:8 is a common next step. Longer is not automatically better. Very long fasts can be harder to tolerate and may be unsafe for some people, especially if you have medical conditions, take medication, are pregnant, breastfeeding, under 18, or have a history of eating disorders.
Key takeaways
- Intermittent fasting means alternating fasting periods and eating periods on a regular schedule [1].
- Common beginner fasting windows include 12:12, 14:10, and 16:8; 18:6, 24-hour fasts, and 5:2 are more restrictive.
- Longer fasting is not necessarily better. Johns Hopkins and Cleveland Clinic both caution against assuming that more hours means better results [1][2].
- Give your body time to adapt. Some people need 2 to 4 weeks to feel more accustomed to intermittent fasting [1].
- Weight change depends on overall intake, food quality, consistency, and safety, not fasting hours alone [4][5][7][8].
- GoFasting can help you log fasting windows, weight, steps, calorie intake, and water intake while you test and adjust your routine.
On this page
- How long does intermittent fasting last?
- Intermittent fasting lengths compared
- How long should beginners fast?
- How long should you try a fasting schedule before adjusting?
- How long does it take to adapt or see results?
- How long is too long to fast?
- What can you eat and drink while intermittent fasting?
- Safety: who should avoid intermittent fasting?
- FAQ
How long does intermittent fasting last?
Intermittent fasting does not have one fixed length. It is an eating pattern that alternates between times when you fast and times when you eat [1][2]. In practical terms, a fast may last overnight, most of the day, or a full 24 hours, depending on the method.
The most common daily methods are time-restricted eating schedules. These are written as fasting hours followed by eating hours:
- 12:12 means 12 hours of fasting and 12 hours of eating.
- 14:10 means 14 hours of fasting and 10 hours of eating.
- 16:8 means 16 hours of fasting and 8 hours of eating.
- 18:6 means 18 hours of fasting and 6 hours of eating.
There are also weekly methods. The 5:2 approach usually means eating normally on 5 days and using 2 lower-intake days in the week. Whole-day or 24-hour fasting methods involve going much longer without food, often from dinner one day to dinner the next day [2][8]. For beginners, the better question is not "How long can I fast?" but "What fasting window can I repeat safely?"
Intermittent fasting lengths compared
Use this table as a starting point, not a rulebook. The same fasting length can feel easy for one person and too restrictive for another.
| Method | Fasting length | Eating pattern | Beginner fit | Watch-outs |
|---|---|---|---|---|
| 12:12 | 12 hours | 12-hour eating window | Often a gentle first step for beginners | May feel too gentle if you already skip late-night snacks |
| 14:10 | 14 hours | 10-hour eating window | Strong beginner option | Still requires regular meals and hydration |
| 16:8 | 16 hours | 8-hour eating window | Common next step | Meals can feel rushed if the window is too short |
| 18:6 | 18 hours | 6-hour eating window | Better for experienced fasters | Harder to eat enough protein, fiber, and total food |
| 24-hour fast | About 24 hours | Usually one full day or dinner-to-dinner | Not a beginner default | May cause fatigue, headaches, irritability, hunger, weakness, nausea, or trouble concentrating [2] |
| 5:2 | Weekly pattern | 5 normal eating days, 2 restricted days | Works for some who dislike daily fasting | Restricted days can trigger overeating or low energy |
| 36-72 hours | 36 to 72 hours | Extended fast | Medical guidance recommended | Longer fasts are not necessarily better and may be dangerous [1] |
12:12 fasting
A 12:12 schedule is often the easiest place to begin. You might finish dinner at 7 p.m. and eat breakfast at 7 a.m. It often works by reducing late-night snacks rather than forcing a major lifestyle change.
14:10 fasting
A 14:10 schedule gives you a longer overnight fast while keeping enough time for normal meals. Cleveland Clinic describes 14:10 and 16:8 as common and often safer bets for many first-time fasters [2].
16:8 fasting
A 16:8 schedule is one of the most common versions of intermittent fasting. Johns Hopkins describes a daily approach that restricts eating to a 6- to 8-hour period, and 16:8 fits that pattern [1]. It can work well if you naturally prefer a later first meal.
18:6 fasting
An 18:6 schedule is more restrictive. It leaves only 6 hours for all meals, which can work for some experienced fasters but may be too tight for beginners, active people, or anyone who struggles to eat enough during the day.
24-hour fasting and 5:2
Some intermittent fasting plans use 24-hour fasts or the 5:2 method. These are not automatically more effective. Cleveland Clinic notes that 24-hour methods may bring stronger side effects, including fatigue, headaches, irritability, hunger, low energy, nausea, weakness, and difficulty concentrating [2].
How long should beginners fast?
Most beginners should start with 12 to 14 fasting hours. If that feels stable, 16:8 can be a reasonable next test. Starting with 18:6, 20:4, or a 24-hour fast is usually unnecessary.
Try this simple progression:
- Start with 12:12 for a few days if fasting is new to you.
- Move to 14:10 if 12:12 feels comfortable.
- Test 16:8 only if hunger, energy, sleep, mood, and meals still feel manageable.
- Avoid 18:6 or 24-hour fasts until shorter schedules feel stable and safe.
This approach is about learning how your body and routine respond. A shorter window you can repeat calmly is more useful than a longer fast that leads to dizziness, rebound eating, poor sleep, or anxiety around food.
GoFasting can help you log your fasting window, weight, steps, calorie intake, and water intake as you compare schedules. Separately, pay attention to hunger, energy, sleep, digestion, and whether the plan feels realistic for your life.
How long should you try a fasting schedule before adjusting?
Try one fasting schedule for at least 7 days before making it harder, unless you feel unwell. One week is long enough to notice whether you can finish the fast, eat balanced meals, sleep normally, drink enough water, and avoid overeating.
After the first week, ask:
- Did I complete the schedule without feeling dizzy, weak, or unusually irritable?
- Did I eat enough protein, fiber, and total food during the eating window?
- Did my sleep stay normal?
- Did fasting reduce random snacking, or did it trigger rebound eating?
- Could I repeat this schedule for another week?
If the answers are mostly positive, repeat the same schedule or test one small change, such as moving from 14:10 to 16:8. If the answers are mostly negative, shorten the fast.
For many beginners, the best path is to repeat a manageable schedule for 2 to 4 weeks before pushing longer. Johns Hopkins notes that it can take 2 to 4 weeks for the body to become accustomed to intermittent fasting [1].
How long does it take to adapt or see results?
Adaptation and results are different. You may adapt to the routine before you see a clear body-weight trend.
In the first few days, the main changes are practical. You learn when hunger appears, whether your eating window is too short, and whether your meals keep you full.
By weeks 2 to 4, some people feel more comfortable with the rhythm. Johns Hopkins gives 2 to 4 weeks as a common adjustment period [1].
Weight changes may take longer and are not guaranteed. A 2026 Cochrane review of 22 studies involving adults with overweight or obesity found that intermittent fasting may make little to no difference to weight loss and quality of life compared with traditional dietary advice, and likely little to no difference compared with no intervention or waiting list [4]. The evidence was limited to studies up to 12 months, so longer-term conclusions remain cautious [4].
The TREAT randomized clinical trial also found that time-restricted eating alone was not more effective for weight loss than eating throughout the day [5]. In plain language: fasting hours can create structure, but they do not replace overall intake, food quality, sleep, activity, and consistency.
Harvard Health makes the same practical point: the quantity and quality of what you eat during the eating window still matter, and overcompensation can work against weight goals [7]. Harvard T.H. Chan also notes that research does not consistently show intermittent fasting to be superior to continuous calorie reduction [8].
How long is too long to fast?
A fast may be too long if it causes symptoms, makes nutrition harder, or pushes you into an unhealthy relationship with food. There is no single cutoff that is safe for everyone.
For beginners, 18 hours can already be too long if it leads to dizziness, intense cravings, poor sleep, binge eating, or missed meals. A 24-hour fast is more demanding and may cause fatigue, headaches, irritability, low energy, nausea, weakness, and trouble concentrating [2].
Johns Hopkins specifically cautions that longer 24-, 36-, 48-, or 72-hour fasts are not necessarily better and may be dangerous [1]. If you are considering fasts longer than a normal daily time-restricted schedule, it is wise to speak with a qualified healthcare professional first.
Stop or shorten the fast if you notice:
- dizziness, fainting, or weakness
- nausea or persistent headaches
- unusual fatigue or trouble concentrating
- sleep disruption
- intense cravings followed by overeating
- anxiety, guilt, or obsessive thoughts around food
- menstrual changes
- any symptom that feels unsafe
A 2024 meta-analysis of randomized trials in adults with overweight or obesity found that intermittent fasting was not associated with a greater overall risk of adverse events than controls, but common adverse events included fatigue, headache, constipation, dizziness, and diarrhea [6]. Intermittent fasting may be tolerable for many adults, but side effects still matter.
What can you eat and drink while intermittent fasting?
During the fasting window, many plans allow water, black coffee, and unsweetened tea. Johns Hopkins lists water, black coffee, and tea as fasting-window options [1]. Avoid drinks with sugar, milk, cream, alcohol, juice, or calories if your goal is a clean fasting window.
During the eating window, focus on meals that make the next fast easier:
- protein, such as eggs, fish, poultry, tofu, beans, lentils, Greek yogurt, or lean meats
- fiber-rich carbohydrates, such as vegetables, fruit, oats, potatoes, beans, or whole grains
- healthy fats, such as olive oil, avocado, nuts, seeds, or fatty fish
- enough total food and water to avoid extreme hunger or dehydration later
Intermittent fasting is not a pass to ignore nutrition. If your eating window is mostly low-protein snacks, sugary drinks, and large late-night portions, the next fast may feel harder. Harvard Health emphasizes that quality and quantity still matter [7].
When should you avoid fasting or talk to a clinician first?
Some people should not use intermittent fasting as a self-directed experiment. If any of the situations below apply to you, do not try to solve it by choosing a shorter or longer fasting window.
Avoid fasting unless a clinician specifically guides you if you:
- are pregnant or breastfeeding
- are under 18
- have a current or past eating disorder
- have type 1 diabetes or use insulin
- have repeated low blood sugar or fainting
- are underweight, malnourished, or recovering from illness
Talk to a healthcare professional before trying fasting if you:
- take medication that must be taken with food
- have diabetes or blood sugar concerns
- have a chronic medical condition
- have menstrual changes or symptoms that worsen with fasting
This matters because changing the fasting length is not always the right fix. If fasting causes dizziness, fainting, severe weakness, binge eating, intense food anxiety, or symptoms that feel unsafe, stop or shorten the fast and get medical guidance.
FAQ
How many hours is intermittent fasting?
Common daily fasting windows include 12, 14, 16, or 18 hours. Weekly methods may include 5:2 or occasional 24-hour fasts.
Is 12 hours enough for intermittent fasting?
Yes. A 12-hour overnight fast can be a practical beginner step, especially if it helps reduce late-night snacking.
Is 16 hours the best fasting length?
Not necessarily. 16:8 is common, but 14:10 may be easier for beginners. The best length is the one you can repeat safely while eating enough.
Is 18:6 better than 16:8?
Not always. 18:6 is more restrictive and may backfire if it worsens hunger, sleep, mood, meal quality, or overeating.
How long before I see weight loss?
Give it several weeks, but do not expect fasting hours alone to guarantee weight loss. Total intake, food quality, activity, sleep, and consistency still matter [4][5][8].
Can I do intermittent fasting every day?
Some people use daily schedules like 14:10 or 16:8 [1][2]. Daily fasting is not required and should not be forced if it causes symptoms or disordered eating patterns [3][8].
What is the maximum safe fasting time?
There is no universal maximum. Longer 24-, 36-, 48-, or 72-hour fasts are not necessarily better and may be dangerous for some people [1].
Can I drink coffee during intermittent fasting?
Plain black coffee is commonly allowed during fasting windows [1]. Coffee with sugar, milk, cream, or syrups belongs in the eating window.
Bottom line
So, how long is intermittent fasting? For most beginners, it starts at 12 to 14 hours. Many people later test 16:8, while 18:6, 24-hour fasts, and 5:2 require more caution and planning. The goal is not to fast as long as possible. The goal is to find a fasting length that supports structure, nutrition, consistency, and safety.
If you are testing a routine, keep it simple for the first week, then review what happened. GoFasting can help you log your fasting window, weight, steps, calorie intake, and water intake so you can adjust with clearer information instead of guessing.
Medical disclaimer
This article is for general education only and is not medical advice. Talk with a qualified healthcare professional before starting or changing an intermittent fasting routine, especially if you are pregnant, breastfeeding, under 18, underweight, taking medication, managing diabetes or another medical condition, or have a current or past eating disorder.
References
- Johns Hopkins Medicine. Intermittent Fasting: What Is It, And How Does It Work? Updated April 7, 2026 https://www.hopkinsmedicine.org/health/expert-qa/intermittent-fasting-what-is-it-and-how-does-it-work
- Cleveland Clinic. Intermittent Fasting: 4 Different Types Explained. Published January 26, 2026 https://health.clevelandclinic.org/intermittent-fasting-4-different-types-explained
- Mayo Clinic. Intermittent fasting: What are the benefits? Published March 8, 2025 https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/intermittent-fasting/faq-20441303
- Garegnani LI, Oltra G, Ivaldi D, et al. Intermittent fasting for adults with overweight or obesity. Cochrane Database of Systematic Reviews. 2026. DOI: 10.1002/14651858.CD015610.pub2 https://www.cochrane.org/evidence/CD015610_intermittent-fasting-traditional-dietary-advice-or-no-treatment-which-works-better-help-adults
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Internal Medicine. 2020;180(11):1491-1499. DOI: 10.1001/jamainternmed.2020.4153 https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2771095
- Zhong F, Zhu T, Jin X, et al. Adverse events profile associated with intermittent fasting in adults with overweight or obesity: a systematic review and meta-analysis of randomized controlled trials. Nutrition Journal. 2024;23(1):72. DOI: 10.1186/s12937-024-00975-9 https://link.springer.com/article/10.1186/s12937-024-00975-9
- Harvard Health Publishing. Should you try intermittent fasting for weight loss? Published July 28, 2022 https://www.health.harvard.edu/blog/should-you-try-intermittent-fasting-for-weight-loss-202207282790
- Harvard T.H. Chan School of Public Health, The Nutrition Source. Diet Review: Intermittent Fasting for Weight Loss https://nutritionsource.hsph.harvard.edu/healthy-weight/diet-reviews/intermittent-fasting/