Not sure if you were only polling Amercian schools but thought I would give you our break from Australia. We are here from 12/12 until 25/1 during our school break. International travellers will all vary but we add to the busiest time of the year. Our daughter school finished on 12/7 and she doesnt go back until 31/1 so we were 1 day short of 8 weeks.
You will find a lot of Australian's at Disney at Christmas we have an extremely long break compared to the American breaks people have posted here.
Depending on wether its a goverment school (state school) or private or religous school it does vary sometimes by a week or more. Each state is also different. But generally it ranges between 6-8 weeks.