I suppose that your length of stay would depend on the people that you're going with and how much that you wish to see and do.
Personnaly I would love to stay for 14 days and 13 nights but my wife, child, and work schedule prevents me from doing so. Two full weeks would definitely allow for multiple unrushed days at each of the 4 theme parks, a day at each water park, a few days of just relaxing at your resort pool, and a few days for extra curricular activities such as golf, water sports, horseback riding, dinner shows, behind-the-scenes tours (my favorite), shopping (my wife's favorite), etc.
The longest that I've ever stayed has been 7 days and 6 nights. These have been with my wife or friends and I did not want to run the risk of getting them over-Disneyfied. My wife likes Disney World, but doesn't love it like I do.
Our next trip will be this November and it will be our 3 year old son's first trip. We're only going for 5 days and 4 nights, as we don't want to push our luck with a 3 year old. I'm certain that we'll be increasing those length of stays as he gets older though.
Who knows? I may just get to stay for those 2 full weeks at some point in the future!
Scott