Cedar Point takes at least two days. You can barely grab all the credits in one, and that's assuming you buy a fast pass and dash around the park. With two days you have just enough time to hit up everything and ride some of the biggest rides a couple times. I spent a couple days there two summers ago, and even with a premium fast pass on one of the days it was tough to get my fill. Just so much coastering to do there. I stayed at one of the cheaper CP hotels just down the causeway from the park... Breakers Express I think it's called. It was very nice, and the package deal was pretty good - especially with the morning bonus time on some of the coasters.
I managed to do King's Island in a day when I went three summers ago. I had to buy a fast pass there as well though. I could see spending two days there, but the coaster collection isn't quite as good as CP (IMO), so I didn't feel like I had to pry myself away after a full day.
Worlds of Fun isn't that big of a park and a single day is enough.
Haven't made it to SFGA yet, but it's on my short list.