I do OMAD w/48-72hr fasts every other week. I also eat under 20gms total carbs. I fast for health, not weight and feel very good when I stick to that program. If I feel off or have been lax, I will add in an xtra extended fast just to get back on track.
Work your way back up to it so it is sustainable!!
Depends on your fat %
The less you weigh the less you can fast.
The body needs to eat something. The fat you have stored or the food you eat.
For medical reasons you can fast until your body fat is really low. I have done 7 days fast to lose weight and control my skin condition.
If you have less then 9% body fat I would consult someone that is not on Reddit.
If your overweight you can easily do 3/1 split easy with just plain water. If you wanna fast longer you need to add salt at least to your water.