depends on how healthy you are. when you fast you are putting tons of stress on your body which will make it stronger. during a fast your adrenaline, blood pressure, etc goes up.
so if you are out of shape i would first do a 24 hour fast from dinner to dinner. once you hit 16-18 hours that is where the benefits kick in so 24 hours is good for your first time
if you are in good shape shoot for the dinner to dinner but see if you can skip dinner and go to bed without eating. then the next day push through.. once you hit day 3-4 it is easy as hell to fast as long as you stay hydrated. your appetite will be gone. my first prolong fast was 5 days and i could have kept going. my longest was 19 days
https://www.evolutionary.org/my-19-day-water-fast-story/ you can also check out my Ebook in my signature on fasting protocols
every person on here whether you are skinny, fat, meathead, pro, etc. should be doing some sort of fasting. it will boost your immune system, gut healthy, kill cancer cells, improve your good cells, improving insulin sensitivity, etc. it isn't just about 'losing fat' like some people think
you meatheads on here who need to eat every 2 hours should atleast do a 24 hour fast every 2-3 weeks to detox and boost gut health. plus when you fast your old and weak muscle tissues die off, so that you can grow new ones. that is why you will actually grow MORE MUSCLE over time when you fast!