How many days you want to go can also depend upon when you are going. It you are going in the off season, the hours are shorter, but lines can be short so you can ride more rides.
I like at least 4 days. One thing you might consider, depending upon your flight plans and when you are going, is whether you might want to go to the park(s) the morning after your last night. If you are going during the busy summer season, DL may open at 8AM. If you aren't flying out until say, mid-afternoon, you can go to the parks early and ride several rides before your hotel checkout and/or trip back to the airport. (The hotel will hold your baggage). I think the 4th day costs around $20/person. Just a thought...
Then again, if you have a little time, but not enough to justify going to the park, you might schedule a nice breakfast at GK the last morning.