We were just there at the end of July. We spent 5 days there. One full day at Colonial Williamsburg, and that was plenty for us. As Allie322 said, the villagers were not that interactive, and many of the buildings were closed up. My kids are 15, 11 and 8 and didn't care for it much at all. Part of the problem was that it was very hot and humid though.
We spent the other 4 days at Busch Gardens and Water Country, and EVERYONE loved it. We never made it to Jamestown and Yorktown because we had just come from 10 days in Washington DC and Gettysburg and my kids had had it with educational stuff. I would like to go back someday and see them though.
If you do go to Colonial Williamsburg have lunch at The Cheese Shop. They make excellent sandwiches there.