When I was running the WDW Great Race in December (it's a RADP/Mousefest thing), during the course of the day we hit all four parks plus a number of the hotels.
The race ran from 8:30A-8:30P.
We visited Magic Kingdom twice. The first time we power-walked to Small World, rode it and left. So that was maybe 25 minutes or so. The second time included a ride on Splash and a pit stop, so that was more like an hour and 45 minutes.
Epcot later on in the afternoon was essentially a power-walk from the front gate to International Gateway with a stop to watch O Canada.
The Studios was a whirlwind puzzle involving all of the survey markers in the park, that was maybe 25-30 minutes.
Animal Kingdom was te shortest visit of the day. We were in and out in 10 minutes, which involved a power-walk from the front gate to the Cretaceous Trail in Dinoland, get our next clue there, and then back out to the bus stops.
Add all that together, and it was probably about 3.5-4 hours total spent in all 4 parks.
Other locations visted included a walking tour of the menus at all Downtown Disney table service restaurants, a tour around the lake of Coronado Springs, trivia in the lobby of AKL, running around the MK area resorts and the Epcot area resorts, and a quick dash into Riverside. And unfortunately for my team, the Race ended with a too-slow bus ride to the finish line at Saratoga Springs... (Who knew it was such a short walk from the West Side to Saratoga?) :-(
-Rob