As a benchmark, on our last trip (2 adults, 2 small children, five full days) in October (low-moderate crowds), we did the following, using four morning and three evening touring hours, per day:
Park (percentage of available attractions/shows/characters experienced) - # days:
Magic Kingdom (95%, favorite attractions 2-3x) - 2.0 days
Epcot (90% of Future World, 80% of World Showcase, favorite attractions 2-3x) - 1.5 days
Hollywood Studios (75%, TSMM 2x) - 1 day
Animal Kingdom (75%, favorites 2x) - .5 days