If I could afford - both in terms of time off and money - to stay in the area for 2 weeks, I would. That would not only give me a good chunk of time for the theme parks and the water parks (maybe an afternoon at DisneyQuest, but as they continue to do nothing new with that space, the less it interests me), but would also give me plenty of time to hit US/IoA and Sea World, plus plenty of down time to rest and relax.
I've also done trips that include a few days "post-vacation-vacation" at Disney's resorts in Hilton Head & Vero Beach. I find Hilton Head far more pleasant, not to mention more convenient if we're driving to and from home.
However, 9-10 nights tends to be the best I can swing. And as my daughter gets older, I'm sure her needs and tastes will dictate where we go, when we go and what we'll do.