TOTALLY depends on you. Best bet is a park hopper, so you can freely come and go on whim.
We did AK in about 3 hours, and probably won't return again until their new ride opens. Just doesn't interest us much...
MK used to take a long time, but now that we're all adults, we hit our faves and skip most. I can see that families with children would spend the bulk of their time at this park...
We could spend days and days at Epcot and not get enough. Its my fave. The World Showcase, in itself, could take me a full week to savor... Add to that the other things, places to eat, great shopping, and diff performers, and it takes a while. Plus, I rarely shop at the other parks since most merchandise is repeated often, but in World Showcase, the items are unique. Takes longer to look =)
MGM takes a day, we love to repeat TOT and RR several times in a day, plus the shows are staggered. We also enjoy eating a sitdown at this park, which takes time.
Good luck!