5 days is longest I’ve done. Day 2 and 3 are hardest. Day 4 and 5 are much easier. Eating ketogenic in the several days to weeks leading up to it makes the fast much easier. When doing it in the summer and sweating a lot from work outside I had to add salt to avoid passing out. So now routinely will eat/drink more salt if fasting. Some vitamins inhibit autophagy so I stop vitamin supplements with fasting. Jason Fung is the person I think has most helpful info on the topic. Depending what meds people are on they may need to decrease/stop them in order to make fasting safe.