Think about it, If your trying to market a dlc, Would you quote the time it takes to finish the main quest 10-20 hours, Or would you say that doing everything in the dlc would take 20-30 hours?
Its going to be 10-20 hours for the lot, This is Todd howard, He wouldnt undersell the dlc, Infact we all know he likes to overhype features, So I wouldnt be surprised if the 10-20 hours includes replaying the other faction.
Edit: lol a sad thought that would be just like todd howard, 10 hours if you play dawnguard once, 20 hours if you play again with the other faction. We all know that the factions will have mirror quests to eachother like the civil war. With slight changes.