Theoretically you could fast to zero fat stores. That is literally what the stores are there for, your survival when going without food. When we fast though it is with a reason in mind, either inability to find food for some reason, lack of hunger due to overeating or to some healing crisis instigating the need for detoxification. Typically whatever the reason we'd usually have enough stores left when finishing our fast to at least go out and climb a tree to find some fruit to break our fast, and maybe walk a few miles to get to the tree as well. Wild animals that fast still need the energy to go hunting for their food after their fasts are done so they need to be at full strength after fasting, not weakened to the point where they can hardly move.
So hunger should return when you have finished detoxing or when you have nearly depleted your stores but still have enough left to go out to break your fast. There is no set time for this to happen. Experienced fasting supervisors seem to be able to predict this pretty well from what I've read but that sort of thing is well beyond me. I myself never aim for a number of days but always my intention is to just fast as long as I can at any given time.