I typically go about 12 hours on, 12 hours off. Sometimes I go longer without eating, sometimes shorter. The strict LeanGains approach is 8 hours on, 16 hours off. I'll vary things depending on this strange experience I sometimes have called an appetite.
I always make sure I've broken my fast before training, even if that just means having had a glass of milk (which I actually find is just about perfect for me as far as pre-workouts go). I won't attempt anything more intense than walking with a bit of direction without having eaten anything beforehand -- intense activity requires more glycogen, stored in the blood, which there's not a lot of when you're fasted. Good way to go unconscious, as I've learned the hard way. If passing out isn't bad enough on it's own, that's a step in the direction of a hypoglycemic coma, which can kill you. Not worth it in my books.