When you say 15-20 hours Im assuming you are talking about more than one playthrough? Cause Im on my first play through at chapter 11 with only 7 hours clocked.. and im on hard. Im not rushing, an amazing player, cheating, or using guides, etc, so it seems more like an 8-10 hour game... which is fine. Its all quality
Although I dont see myself playing it again on the impossible settting. There have been a few points later in the game where I wanted to scream lol. The game can get frustrating in the sequences where they overwhelm you with 5-6 monsters at a time, and then they add more. There is one area.. I think chapter 9 or 10 where I was thinking "are they fucking for real!!" but after I did it I had an awesome feeling. hahah but Im not doing that again.