First run lasted 13 hours on nephilim with B or better grades, but note that completion % (keys, secret missions, lost souls) were low all along (some levels require to come back and replay after you got Aquila or some specific key). Game has quite a bit of content to look out for, and how much of it you collect acts as a multiplier on the final grade for the level.
Also, one last note: I didn't play the demo, but In the final code the grades definitely don't go up like a breeze as in the video you linked before. I think I got SSS like 10 times in the whole game.