It totally depends on your group and what their interests are. We are all adults, and we spend 1/2 day at MK now usually. My DH is less than thrilled with the rides there, so we hit the highlights and head out.
We also can do AK easily in 4 hours or so... just doesnt interest us much. We do the highlights, and head out.
Epcot is a longer one... 2 days minimum for us. We enjoy browsing the countries, eating, the new rides, spending time in the Aquarium, etc. We find the exhibits there take alot more time than riding a ride at MK.
MGM is my DH's fave, because it has several cool rides that he loves. HE could spend a full day or more there, just repeating things and eating at the nice places.
So you see.... its diff for everyone. My suggestion would be to buy the park hoppers and play it by ear. We never stay in one park all day, we enjoy hopping each day. Maybe dont buy your Bush Gardens tickets yet, and play it by ear for once you get there? That way you know you have one extra day to play with if you decide you'd like to stay longer at WDW =)
HAVE FUN!!!!!!!!!!!!