I've done 3 days many times. The first time was in jail, they just didn't feed me anything for over a day as we were traveling in cop cars and I realized after a while "man, I don't even need to eat!", after about 24-36 hours your "hunger hormone" or whatever just stops working and the craving for food dies, in my experience. That inspired me to try again and again on my days off work, and I would go 3 days without eating all the time. Apparently this is healthy because it "regenerates neurons in your brain" or something, they found that people that do this kind of fasting live longer, they even have acronyms like ADF (alternate day fasting) or 5:2 fasting, etc.