Hi SeanMD90, from me the short answer is….who really knows?. Since the 1st April I have done 5 fast days including two water fasts. My first was a water fast of around 30 plus hours leading into a 2nd 600 cal fast day. So Monday – water fast, Tuesday, from 1.00 pm ish a 600 cal fast day. I did a 2nd water fast of around 30 hrs on the Friday. I felt fine, very focused and mentally alert during the water fasting periods and found it easy to do. You can Google water fasting to your hearts content whether a 24 hr fast or a 5:2 fast which is water only or includes calorie restriction. It falls to your own preferences and what suits you. I believe that there is sufficient evidence emerging that generally, fasting has positive internal health benefits.
If a 24 hr fast suits you then “Just do it”. We are all different and the 5:2 is very flexible, aimed to suit our individual needs provided we try and stick to two 500 or 600 cal fasting days. Experiment,try different things until you find what suits you.
Whatever fasting “method” I do within the 5:2 way of life my eating window is a 6 hour period, some have 8 hrs, do what suits you.
Good luck.