About 5 hours.
If that was the time most users experienced, then I'd fully agree with the thread. I haven't seen anyone else say it was that short though. I did a playthrough as evil just focusing on the main missions and it was well over 5 hours. As I said I'm doing a second playthrough as good and this time focusing on side activities. It'll be about 20 hours for this playthrough.
Did you not upgrade any of your powers?