I typically do either 2, or 4 days at Epcot. Future World basically is two sides, one being The Seas, The Land, and Imagination, the other being The Universe of Energy, Test Track, and Mission Space.
We will do one side of Future World in the morning, and then spend our afternoon wandering World Showcase, then the following visit we do the other side of Future World in the morning, and again spend our afternoon in World Showcase. That system seems to work pretty well for us.