There are a lot of things to consider when deciding how long to spend at WDW. Money, weather, the travellers etc.
Our last holiday we spend 8 nights on site at WDW and we found that it was no where near long enough, thats why we are going again this year. I was not prepared for how tired the children got (even though they were aged 8 - 13) the heat really sapped us of energy as well. Even though we went when it was really quiet and we hardly ever had to wait in a line (except for toy story mania) we did not get to see or do all the things on our must do list.

It also gave us no time to venture outside of WDW. Also will you be going to the water parks as that will take time away from the other parks. It was our first experience with water parks - we have one pool here with a swirly slide but thats it. And we fell in love so we spent 2 full days at the water parks and plan to spend more time there at our next trip.
This year we will have over 2 weeks just at WDW and also 6 nights at a hotel at universal studios and I am sure even then we will run out of time and not do everything we want to do.
Have a look and plan out the things you really want to do at WDW, which parks, what rides, particular restaurants and try and do a time frame. That will help give you an idea on how long you might want to stay there.
good luck, I am sure which ever way you go, it will be a magical holiday.
