We love long trips with our little guy, since it gives us plenty of time to relax, take things slow, enjoy the pool at the resort, etc.
We took him for his first birthday and stayed 8 nights. We took him again at 20 months and stayed 10 nights. Both trips were fabulous! He's quite a traveler and has been on lots of other trips, too. He never has any problems. That probably depends a lot on the personality of the child, though. Ours happens to be very laid back, easy going, and flexible.
One thing I will say is that, since we started traveling with a baby, we have been renting points and staying at the DVC resorts. It makes such a huge difference! It's wonderful! We can usually get a one bedroom villa for less or about the same as we would pay for a regular deluxe room. What a deal! We love having the extra space for him to play, the ability to put him to sleep in one room while we stay up in another, the washer and dryer, the kitchen, etc. Now we're spoiled and will only do DVC resorts when we travel to Disney with the little guy. I think it makes the long stays much more comfortable. Luckily, renting points makes that very affordable for us until we can get it together to become members ourselves.
By the way, it looks like our stays will overlap! Maybe we'll see you there!!