I've heard some people say 20 hours, some people have said as much as 40. It really depends. I managed to make it through the first full "region" in 7 hours, and I know that there are at least three different regions in the game.

And no, I'm not done with it yet. I've been dividing up time between this and Baten Kaitos. I expect to have RE4 finished off sometime over the next two or three weeks.