It’s easy for me. When I stop enjoying it, the end is very near
But I never needed it for some specific reason, that could have made a big difference.
But if my body tells me it’s enough (I get annoyingly hungry, weak or disoriented), I always stop. I dislike ignore the signs of my body, it knows WAY better what it needs (it just have a not so right idea about it sometimes, it is like a spoiled kids a bit but it has the power to make me miserable so I better don’t say no to it. sometimes I do).
Not everyone handles EF well even when the time is right (as I had no problem one time and couldn’t do it another time). I suspect that even my fat body with plenty or reserves can’t handle a longer one but I don’t want longer ones anyway. 48-72 hours would be lovely and enough. My longest was 5 days, just water (and probably tea. my coffee times came later). I had no idea about electrolytes at that time, it was funny… I continued when I got weirdly dizzy as it wasn’t unpleasant. I continued when I started to think about food in some obsessed way (but I had no urge to eat. it was all mental). I have stopped when I got hungry (but only could eat a few bites, normal feeding came the next day). So I only did it when it was easy (in my determined, curious, stubborn, not tempted state, that made it trivial) and I felt well enough. I don’t even try to break through something serious, I know it wears me out in no time anyway and I don’t do suffering. Maybe for some very sure health benefits but I would feel unlucky to have that situation. I don’t think fasting is strictly needed for me but it still have some benefits and anyway, I could fast easily (but I didn’t choose the time usually, it just happened) on high-carb, I could do it (with difficulties) on keto, I won’t accept carnivore made it impossible for me for ever. But I am against forcing myself. As I wrote, it can be different for others due to a different personality/attitude or different, very good and important reasons for a fast.
But 96 hours is pretty long, I don’t think stopping is a bad idea at this point. I would already worry about my muscles, I simply can’t imagine our not very tiny protein need can be met without food at this point. Of course, the muscles can be rebuilt and in some cases, the benefits may overpower the loss but I still don’t want to lose them in the first place or just in very minimal amounts. So I do the shortest EF when I do any.
Thankfully the already built but lost ones are easier to rebuild than going further than ever… IDK if it is affected by aging too though (sure, if we lost some that is beyond what is possible at a later point, we can’t get that back but otherwise, when one is far from the upper limit? not like the limits are necessarily fixed but I am out of my depth here).