Sure, you can beat this game in a day - there's almost no doubt about it.
But then you're undoubtably missing out on numerous subplots and sections. Like a comment I read elsewhere that proclaimed he beat all 3 endings in under 10 hours (there's 5 endings).
I think I spent well over 4 hours in Seattle alone, and my total play time the first time was probably around 30. Course I would spend half an hour just trying to figure out if there was a way to do stuff like, free the guy from the SSC jail.