I bought 4 day 2 park tix when we went for the first time last year, and we were able to do everything we wanted in 2 says. The other days we were only there for a few hours to see/do things again.
That said, neither of my boys will do roller coasters, so we were there mainly for HP (one did FJ, neither would touch the Dragon) and the tamer rides. We also lucked out and got a coolish, rainy week, so we did everything but the coasters at USO before 4pm the day we were there and didn't go to IOA.
If you are thrill ride types (like me), I would think at least 3 days, with day 3 for anything you missed or wanted to ride again.