I would allow 2 days for Epcot. For starters, it's very large and there is a lot of walking to get from place to place. Future World can take almost a day by itself, depending on how long the lines are and how much of the secondary stuff you do (every pavilion has its main attraction (ride), most of them also have secondary attractions at the exits. Here's a tip: if you've got someone in your party who is not one to ride the big stuff like Test Track or Mission:Space, have them enter the pavilion through the gift shop/exit. There they will find fun stuff to do while they wait for the rest of the party to get off the ride)
World Showcase has different entertainment in each country - be sure to pick up a times guide as you enter the park to know when you might catch something fun! Canada, China, & France have movies. Mexico and Norway have rides. The American Adventure is not to be missed! The rest are primarily shops and restaurants, with a few exhibit galleries. All of which are great fun to walk through. And be sure to stay for Illuminations: Reflections of Earth at closing!