I've seen a lot of these threads pop up now that Dawnguard's out for Steam, so I'll just throw in my long-winded two cents.
I find it really hard to judge how many hours I got out of Dawnguard. I made a new character for it, a High Elf Mage/Assassin that worked so damn well it was beautiful, and he became a vampire. I took my time doing the main quest, I'd say it took me maybe 15 hours to do that (but I really just made up a number.)
The main thing Dawnguard did for me was it got me back into the game. I didn't just do the new quests; I did the College and the Civil War again, the Thieve's Guild and Dark Brotherhood for the first time, I became a real estate mogul, a daedric artifact collector, an alchemist, and did a bunch of old side quests I hadn't ever done before. And I did all this while wearing the badass Vampire Royal Armor (decent light armor, +125% Magicka regen), the Ring of the Erudite (+100 Magicka in both elf and vampire lord forms, HOLY ASS) and abusing my vampiric powers (They made vampires actually viable, people don't turn hostile when you're stage four!)
All in all, I played that character for 100 some hours, and then started another one I played for about 20 before I moved on again. And I'm probably going to go back to one of those two, or both. Skyrim is just my kind of game, though, so your mileage may be significantly lower.