It depends too on how interested you are in the subject matter. I have been interested in Titanic history for years, even before the wreck was discovered, so artifacts and things like that really catch my interest. The guides - not so much. There were a few times that I really wanted to step in and say "No, no, it didn't happen like that!" but overall, it was a good experience. I think I spent at least a couple hours there.
If you've just seen the movie, and have a mild interest in the subject, an hour or so will probably be plenty.