From your username I assume you have kids? Are they used to traveling? Have they been to Disney before?
We ended up with 6 (maybe 6.5) days in the parks on our first big trip last September. Then DS and I did 5 more in December (DH was there for 3). Since that trip wasn't really yearned for by DS, he wasn't totally into it, and b/c of very different touring styles I caused DS and DH to burn out. oops!
So then I did 1.5 days ALONE in January, so I don't have to be a park commando next time I go with them.
I think it's obvious that I, personally, could stay for weeks and be fine, touring all day every day.

But DH and DS need a shorter, intense, stay, or a longer, more relaxed stay.
So you can see, there's no one answer even inside of one family.
Where are you coming from? What time will you get in? Is there a time difference that will help or hurt that traveling day, or that first day? If you're coming from the east coast you'll probably want to take it easy on travel day, but that next morning you'll be up bright and early b/c it will be "later" to you than it actually is.
There are so many variables.