I personally recommended that you stay for a minimum of 7 nights especially if coming from abroad.
The first day will pretty much be all spent on a flight to Orlando, travel to your hotel and stocking up on food at the nearest supermarket. The second and third days can be spent at the Magic Kingdom. The third and fourth at Epcot and Animal Kingdom. The fifth at Disney MGM studios. And that's pushing it. You should really be spending a minimum of 2 days per park (more if you plan on repeating rides). As well as that you'll need at least 1 day per water park (there's 2 Disney ones). There's also Downtown Disney to visit as well as two Universal Theme Parks. Then, there's the day you leave when you'll be expected to be at the airport a minimum two hours before your flight leaves. This shows that spending at least 9 nights is the best option, with 12 or 14 nights being ideal.
You may also notice that there's not a big difference in price between staying for one week and staying for two, though you do have to take food into account and park tickets. At the time of writing, Disney is offering 14 days of tickets for the price of 7 with unlimited park entries to all parks and hopping is included. After checking booking prices - staying at Disney for 7 nights in August 2010 for 3 adults and 1 child would cost £3900 and for 14 nights would costs just £4550. That's just £650 extra for a whole extra week. Combined with the current deal fo 14 days of park entry for the price of 7 - this is a cracking deal. The hotel works out at just £25 per person per night. NB: 21 nights costs £5100. Flights from London Gatwick.
