i am almost through with the game on medium difficulty and have clocked in around 8 hours so far. i have collected three quarters of all collectibles and played 5 out of 7 mission and liberated all but two garrisons. i think it's well worth the 15 bucks.
I did everything in it and it took me seven and a half hours. If you were to just run through it, it's probably three to four hours, possibly. The game's worth buying. If you play on hard though, I'm sure it'll be up to ten at least.