Well this is something that depends on so many different variables, most of all cost.
If I had unlimited funds and no need to go back to work, I'd love two week trip to spread things out, take time to relax, and enjoy all the World (and maybe beyond to the rest of the Orlando area) has to offer. But at the same time, with school age children, finding the right time to do 14 days would be a challenge. Obviously, it's much more crowded when the kids are out of school (and in the summer can be a bit too hot), and I much prefer going when crowds are lighter. While I'll take the kids out of school for a day or two, I won't for a full week.
But budgets are limited, I do have work, and we also like having time to visit other places during the year. Last time we went for 4 nights over a fall school break and it was certainly a full trip, and we ended up not able to get to Animal Kingdom. Our next trip will be 6 nights, also over a fall break. That gives us the flexibility to visit MK on multiple days and take in each of the other three major parks - essentially experiencing all of our must-see park attractions/entertainment plus a good deal more, and still have some lazy pool time mixed in.