When I get really dizzy and or when I feel extreme thirst.
My rule is basically, if I feel thirsty I’ll wait 30 minutes and see if I’m still thirsty. If I’m still thirsty I’ll meditate on it a bit and see if there’s something besides thirst going on.
I always break on extreme thirst, I’m not messing around or trying to break a world record. I’m looking for healing.
The last dry fast I quit early was because I kept getting very light headed almost / blacked out a few times when I stood up. Finally decided that was a bit much and I should drink / eat.
Fasting is a marathon - not a sprint. Go slow and steady and walk (i.e., drink it eat) when something feels off.