6 hours sounds a little bit short. If reviews are accurate, they indicate that the marine campaign is the longest, with the most substance. Assuming your average gamer is as capable at FPS at the OP, it may only take a weekend to finish the whole game.
IF I get the game, I'll most likely be buying it for the single player initially at least until I get a new graphics card. I hope I don't get through the campaign(s) as quickly as the OP.