"Those who have it...How long is it?" -That's what she said. Anyways... I imagine it's long. Some have said 90 minutes per mission, but that seems insane. However you can select checkpoints from the replay screen... so why not?
I wouldn't expect the two campaigns to be more than 5-6 hrs each. After all this is primarily a shooting game with some rpg elemets. Unlike Dragon Age, Witcher, Neverwinter Nights 2 which are based more on actual game length.