‘What’s your body fat percentage?’ is a key datapoint basically (I am not a doctor or researcher disclaimer). Back in the day my afternoon blood glucose was 260 down to 190 after 5 days water+black coffee fast. Was 110 after 8. None of those are good numbers. Don’t be fat and type 2 diabetic and 5 days is probably grand. If you’re 10% body fat male longer fasts probably aren’t as beneficial.
Yes. Your body clears out it’s quick energy supply after about a day, and starts burning fat (ketosis). After about three days, you enter autophagy, which is cellular repair. I fast for about 5 days specifically for autophagy, because I have illnesses in my family that I am trying to avoid, and staying in the autophagy state is one tool among many that I use to lower my risk of developing those illnesses.
It depends on what you’re looking for. The benefits are all “unlocked” at about 72 hours, but those benefits are still intensifying. For example, you’ll continue to get more insulin sensetive, burn fat, have an increased rate of chaperone mediated autophagy, etc. So it depends what you’re optimizing for and how much progress you have to make. You can only get so insulin sensetive.