I fly. If I don't get to heaven, my hell will be roadtripping with small kids.
As many days as you can. Disney is 51 square miles. You can easily spend a day in each theme park and not do EVERYTHING. Then there are two waterparks, minigolf, Downtown Disney, resort touring, swimming, fishing, horseback riding, boats to rent, restaurants to visit. You can stay a month and not get bored, if you turn out to be a Disney person. (Not everyone is, and some people don't like it on day one).
We've taken three trips in the past five years - all a week long - and there are still things I'd like to do that I haven't yet found time for.