As others have said, it depends on several factors - who you're traveling with, what their interests are, what is closed at each park, how often you go to the Parks etc.
When my children were little, we spent a lot of time in MK. As they have gotten bigger, they're looking for things like Everest and Test Track and less for characters and Dumbo. With their ages now (between 9 and 15) we do 2 days in MK, 1 at Epcot, 1 at AK, 1 at HS and usually an extra day to hop and finish the things we didn't get to do at each park. 6 day park hoppers serve us well, but we can also do 5 days without missing much.