Skyrim - Dawnguard Preview, Tidbits
Gamefront has a short preview of Dawnguard, quoting Lead Artist Matt Carofano about the length of the DLC:
“It’ll take between 10 and 20 hours to beat,” Skyrim Lead Artist Matt Carofano told Game Front during an interview at E3 2012. “It’s about as long as a faction line.” So about as long as the Mages College or Companions Guild quests.
Meanwhile, The Official Playstation Magazine UK (catchy name) has some tidbits from the beta, including screens and descriptions of both the Vampire and Werewolf perk trees.
Platform: PC, Xbox 360, PS3