Depends on how much you want to do. I literally could spend all day there (and have many times).
Just a leisurely stroll where you take your time, explore each pavilion, ride the rides in Mexico and Norway, watch American Adventure and Voices of Liberty, do some shopping, enjoy the architecture/detail - will take up a big chunk of your afternoon.
If you make plans to watch any of the movies (at France, China, Canada), see a couple musical performances (Off Kilter, MoRockin, British Invasion), see street performers (chinese acrobats, world showcase performers in England or Italy, etc.), eat lunch and/or dinner, see illuminations, and just generally take advantage of what it all has to offer, you can spend all afternoon and evening there, easily.
To me, this is why Epcot is a 2-day, or at least a 1.5 day park - because if you throw Future World into the mix, there is an absolute ton to see and do.