TrumpetEd

I think a lot depends on the strength of your embouchure. Because of my very heavy schedule, I try to practice at a minimum of twice a day, an hour minimum at each time. This does not include the warm-up. I have a friend who practices three times a day around his meal times and he has incredibly strong chops. Also, many professional players only have enough time each day to just maintain their chops because their already playing so much. I agree with Mr. Laureano though. If you can get in four hours a day, that would be awesome, but the main thing is that you're practicing correctly. An hour of correct practice is better than 8 hours of practice that could be damaging.

Ed Annibale