Todd Howard himself said that Dawnguard is 10-20 hours. How someone can double that is beyond me, if the man himself said it.
It's also been said, in another interview, that 10-20 was the MQ. The Skyrim MQ can be done in that same amount of time. Does that mean Skyrim is 10-20 hr game?
This is TODD HOWARD we're talking about. There's a reason this is a minor meme: http://www.youtube.com/watch?v=6ne1fVhvL00
This man spews more lies out of his mouth than anyone else in the business. If he said 10-20 hours, you wanna assume the 10 hours, not the 20.
You might be right, you might not. But again, this is TODD HOWARD. Very good odds you're setting yourself up to be disappointed if you take what he says and assume HE is providing you with a bare minimum and being modest about the actual size. Nah, that doesn't sound like Todd.