Hi Jonathan!
Intergrading 'down time' is vital when planning a vacation at Walt Disney World. We always make sure that we plan more time in a park than is needed so that we have time to stop and smell the roses, sleep in a few days, enjoy the pool, etc.
That being said, we usually allow 2 days for Magic Kingdom, 1.5-2 days at Epcot, 1-1.5 days at Disney's Hollywood Studios, 1/2 to 1 day at Disney's Animal Kingdom, and a good 3/4 day at Downtown Disney. We are park hoppers, though so our time at the parks (when you look at our itinerary) looks very broken up. If you don't have
park hoppers, you can easily plan other activities such as pool lounging, or a trip to Downtown Disney in place of that remaining day. Also, keep in mind that time of year also plays a role into what you'll be able to accomplish in your day's time. If you are planning on visiting during a slow period, you might only need 1 1/2 days at Magic Kingdom, whereas if you were visiting during a busy period, you might need 2 1/2 (or more).
Have a magical vacation!
Amanda