If you're going when it's busy you'd need 1-2 days for MK, 1/2 day for AK, 1/2 day for a waterpark, 1 day for MGM, and 1-2 days for Epcot. Then you also need a day to relax and do nothing or you'll be miserable from exhaustion.
So I say 5-6 days.
If this helps when I go in July my schedule is
Day 1 - MK
Day 2 - Blizzard Beach
Day 3 - MGM
Day 4 - MK and AK
Day 5 - Epcot
Day 6 - REST and Downtown Disney
Day 7 - Universal Studios
I actually would have liked one more day to relax because it's gonna be tiring doing 5 days of parks with no break, but I have limited financial resources. :lol:
Or if you're going during the off season when you don't have lines and heat to consider you can really fly through everything. My husband and I got there on a Thursday afternoon at the end of September and did every ride at Typhoon Lagoon, Epcot, MK, AK, MGM, and Disney Quest and left Sunday afternoon. But it was quite tiring. :lol: