A lot. That's quite a difficult term for me to get to grips with, because if you deprive an animal of feed, then it will just try to mobilize its body's energy reserves.
The first energy reserve it calls upon is carbohydrates, and that tends to be stored in the liver. If you look at how long it takes for, say, the liver glycogen to be completely exhausted from a bird, we're talking four hours there. If we're looking at sheep, we're talking several more hours. Then, when it's used its carbohydrates, it goes over to fat and has to burn that off. Therefore, it depends upon how thin or well-muscled the animal is as to how it can cope with that. The final energy resource that an animal has in an extreme situation is that it has to try to burn off its protein, whether that's muscle or even the heart protein.