Dont recall what the "typical" length of a prepatch is, but I would be surprised if it is much more than 3-4 weeks at the maximum.
Sure Naxxramas isnt as big as TBC releasing, but it was still a big thing alot of people looked forward to, and a lot of people almost exclusively played classic to partake in Naxxramas. They did release this almost at the same time as SL.