When we were there in 2008 we had 3 days which was perfect! We explored the parks and rode all the rides we loved 2/3 times or 7 times as my youngest DD who was 7 at the time did with Superman because she wasn't tall enough for Hulk
This year we're allowing 4 days just because we are HP fanatics and I'm sure both of my DDs will want to spend the best part of a day at WWoHP
OH! We stay on the property.