I have understood that after 72 hours of fasting the body starts to use muscle cells for energy so we shouldn't do that, but a longer fasting period (once a while) than the standard might be helpful.
My longest fasting window is approximately 21 hours. I just added a few hours to the daily fast, because I wasn't very hungry and was working on something that kept my mind from thinking about food. I still ate pretty much the same during the 3 hours (maybe a bit less) as usual. My usual fast is 18 hours.
I "fear" fasting longer than 24 hours though, because then you need to go to sleep hungry. BTW. If I am fasting, that means no calories intake at all (only water, tea, coffee)
What is your longest fasting period?