When it comes down to it, the optimal amount of time to fast is the amount of time a person can realistically spend in autophagy, considering both physical and mental factors. (In other words, you’ve still got to do life — and that has to factor into your fasting plans.) While a week-long, zero-calorie water fast is fantastic for your health, it’s not realistic for most people with a busy work schedule and family life. Based on my research, a three-day modified fast is the right mix of doable and highly impactful for your health. I find that people fall off and feel like a failure when they attempt 5 days of modified fasting, but virtually everyone finishes strong with 3 days.