The best window is the one that works best for you. The schedule has to be effective, flexible and sustainable. One of the keys to having windowed eating work for weight loss is achieving appetite correction. When appetite correction happens, the individual does not have to track calories or exercise willpower to eat less; he or she simply wants less, and the weight comes off steadily with relatively little effort and little, if any, trouble with feelings of hunger.
Fast-5's 5-hour window appears to be more reliable in achieving appetite correction than longer windows. An even shorter window has been required for some to see appetite correction, but 5 hours works for 90 percent or better of those who apply it with a reasonably consistent schedule.
The 5-hour window is generous enough that a person can eat a meal without feeling like overstuffing is necessary, because there is time to eat later should one become hungry. The window can be moved to any time of day, and it can be moved temporarily to accommodate social occasions. The five-hour window may also be more forgiving of slips and skip days, meaning you can still see progress even if you take a day off the schedule now and then or occasionally extend the window.
The non-weight loss benefits of fasting appear to be tied to keeping insulin levels low, so the shorter the window, the more likely those benefits to accrue, but we do not yet know where the point of diminishing returns is (i.e., is a 2-hour window better than a 5-hour window). Five hours is adequate to achieve some substantial anti-inflammatory effects in many people who have autoimmune or other inflammatory disease. For example, Fast-5 provides approximately 16 hours of low-insulin time per day (assuming one eats at the very end of the window, and it takes 3 hours for digestion to be completed and insulin to return to baseline levels) or 480 hours in 30 days. An 8-hour window provides 13 hours of low-insulin time per day, or 390 per 30 days, a difference of 90 hours or the equivalent of almost 4 additional fasting days per month.
So, as a starting point, the 5-hour window is practical, effective, and sustainable for most people who have tried it, and may deliver greater long-term fasting benefits than longer windows.
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