We did US and IOA for the first time last week after years of only Disney. We felt they were just diff then Disney, but just as impressive in their theming and cleanliness. We stayed at the Royal Pacific in order to have FOTL and DEF recommend it. We put aside two days, and only need half of both to do all we wanted... big diff from Disney there. I could spend weeks in Epcot alone, but quickly did US and was ready to go. IOA only took a few hours, as well. esp with FOTL, we walked right on. Suppose it depends on if you have children that will want to spend time in the kiddie parts, which we skipped, if you do water rides, which we don't, or if you like to ride things several times. DH and DB went back in on day two to ride things again, but they were done with that rather quickly as well.
I would give two days and take your time. =)