I regularly go 36 hours+. I am alternate day fasting for the month of July. That means I eat every other day. I love it, but I am morbildy obese and have lots of fat to feed off. It isn't for everyone. If you are conditioned for it, and seeking serious fast weight loss, it is a terrific tool. Make sure you keep proteins high to avoid muscle loss. Some amino acid recycling is bound to happen. I think it's super healthy. Others simply cannot handle it. If you are prone to eating disorders, please do not IF longer than 16 hours. No one knows exactly what is best for everyone because everyone is different. I need a big calorie deficit to lose weight at pace that makes me feel like this is worthwhile. I do not want to be fat one more day longer than absolutely necessary. My thinking is the unhealthy stuff of crash dieting so it needs to be understood as an accelerated program that is part of a permanently healthy primal lifestyle. For weight loss, I cannot imagine an easier and more effective combo than primal + IF. If you can extend those body fat burning IFs with some extra hours and still feel good physically -- congrats and good luck!