Campaign took me about 12-14 hours with lots of exploring and reading in-game lore. I'm sure anyone can beat it faster but frankly I slowed my pace as the world in the game is absolutely facinating. Matrix type thing, end of the world and all humanity kind of stuff. I highly recommend giving this game a playthrough if you can, especially at this price. If you go into it with zero expectations and have no idea what your getting into, then you'll have a great time. Listen to everyone's story, and keep interacting with the "persons" you meet. You'll get even more out of them.
I probably watched 4 lets-play type runs of the game before playing myself, and I still cried at then end. "This is it? This is how their Earth is now?!" I loved the story, couldn't get enough of it, and each watch of lets-plays and even my own playthrough.. kept catching stuff. What a wonderful and horrific way to bring mankind's story to an end.