The last time I was there was 2007 or 8, but in two days we just barely did everything with a few re-rides here and there. If you really love rides and want to make sure you get everything in with plenty of re-rides, three days is not a bad idea. You won't get bored.
The Fast Passes are worth it to me if the lines for big things are over an hour, considering this is a destination park for me that I visit once every blue moon. I consider the CF system well managed, so I don't have any qualms about taking advantage of it. When I was at KI last year it was a cold, wet Tuesday in early June and the place was just packed so I splurged on one and it was the best decision I made that trip. I rode everything multiple times versus just barely hitting all the major coasters.