Just a friendly heads-up:
Any games can be played through extremely quickly. The original Resident Evil lasts about 12 hours for most people, but it is very well within human limit to finish the game in 6 hours. A speed run, of course, could be finished in something like 2 hours.
But the thing about that game is, it still takes me something like 15 hours to finish years after it is released (I suck, ok?)
It still takes me well into dozen hours to finish most console FPS simply because I'm pathetic at it. However, most people are pretty good with the genre by now and will finish the game in the equivalent of s-rank time in older games. It's not the game's fault--the fault lies with the system that it is on.
See, with console shooters, the game simply cannot be hard. Consoles are horrible to make shooters with. You can't have the game pose a real challenge, or the average players will never finish the game. But dumbing down the game just make it too easy for the veterans.
Chances are if your first playthrough can be done on hard difficulty, you WILL finish in 5 to 6 hours because you're no longer an average player.
And you should also count the time that you have to restart the game. It's probably only 5 to 10 minutes per check point every here and there, but it will quickly add up to a few extra hours too.