From start to finish, and having a good go, I'd say anything between 10-12 hours.
If you're an achievement junkie, then you can easily double or treble that because of a) the collectables are pretty tough to find, b) you get an achievement for doing a "good" ending and an achievement for doing a "bad" ending and c) there's a known bug which could mean you having to go through the entire game again.
I reckon I played it about 5 or 6 times and enjoyed every second from start to finish.