Experts agree that your personal habits will play a role in when exactly certain health benefits will kick in. "There are very few definites here," says Vincent Pedre, M.D., an integrative physician and gut health expert who frequently recommends intermittent fasting diets to his patients. "Where you start seeing those benefits depends on what you eat, how healthy your gut health is, what kind of physical activity you're doing, etc."

For example, letting calories slip into your fasting window, overeating calorie-dense foods like pizza during your eating window, not prioritizing sleep or physical activity, or making one of these intermittent-fasting mistakes could all delay or counteract the benefits of intermittent fasting.

Even if you're formulating the rest of your diet and lifestyle perfectly, it's important to understand that the different health benefits associated with intermittent fasting will likely kick in at different durations of fasting. For example, weight loss may be triggered by a relatively short fasting window, while entering ketosis or triggering autophagy—that "self-cleaning" cellular process that boosts brain functioning and maybe even longevity—may take significantly longer.

Unfortunately, not enough research has been conducted on all the different forms of intermittent fasting to provide any hard-and-fast rules. Based on personal and patient experiences, however, the doctors and researchers we spoke with were able to offer a general idea of how long you may need to fast to reap certain benefits, which we'll dive into below.