Well, maybe I exaggerated a little, but consider this:
In a movie, usually one page from a script will translate to one minute of screen time. So we're looking at approximately 8 hours of cut scenes/dialog. Not bad. I can't speak for par time during the actual gameplay, though. It all depends on the player. My prediction is that the game will eat up roughly 20 hours of your time. Also, some lines were probably written for incidental occasions, because they mentioned that the Ghostbusters will comment and react to your actions in specific ways. That means you won't have a chance to hear everything the Ghostbusters have to say, since it all depends on what you do in the game. Now that I think about it, that should add some good replay value.
Some games are never-ending, and others have stories that spread out for 80+ hours. I think most Ghostheads are pretty experienced gamers. 20 hours will be a drop in the bucket. If this game is all you do in your spare time, then you'll have the story mode wrapped up in two or three days.
The multiplayer should keep you busy, though. There will be plentiful competition available during the first month after its release.