My first trip we had 4 day parkhoppers. We didn't see everything, but we saw everything we had planned to and had plenty of time to re-ride favorites, though at the end of the last day my partner said she wished we had one more day. Based on that I feel that two days is enough time for a good trip if you're not one to worry missing some attractions and shows, three days is enough time to do just about everything you're likely to want, and four days is enough time for re-rides and to do a lot of activities that might be lower on your priority list. Five days would be handy if you want to have some partial days in there to mix things up. If doing four or five days I would suggest adding a day of something else in the middle to rest feet and get away from the crowds.
We are two adults so we could move quickly and easily, didn't need nap time, and didn't need to make time for any of the kiddie rides. We had short hours when we went, but we stayed open to close. If you need to accommodate a range of different ages and interests in your group, or if you need breaks or early bedtimes, definitely err on the side of extra time. However, also keep in mind that you can add days while you're there, provided your other vacation plans allow that kind of flexibility.