Ideally, I would say that you can hit all the major attractions (and most of the rest) in 2 days each at the Magic Kingdom and Epcot and 1 day at MGM and Animal Kingdom. Obviously, it all depends on what you want to see and how the showtimes work out, but I think this is the general consensus. Hope this helps.
By the way, I would work in a day of R 'n' R every 2-3 days, because those parks take alot out of you.