I guess you can beat Mass Effect in 20 hours if you focus ONLY on the main quest and do absolutely nothing else. But there are tons of side missions to do and character backgrounds to explore.
Like STICK said, it focuses much more on the story so don't expect to explore places like you can in Fallout. But the story is really good so that's not a problem. Do the side missions, do the unique character missions, talk to everyone to hear all the dialogue and you'll spend a good amount of time with the series.
And I know you're on PS3 so get the Mass Effect Trilogy which has all 3 games. I think that's the only way to get ME1 on PS3. I never played the first game and feel like I missed out on a lot so make sure to play it.
EDIT: Actually, I think you can download ME1 from PSN so it may be cheaper to get it there and the other 2 games used.