JustThatGamer on 23 January 2014
Normal difficulty setting on my first play-through:
GoW1 took 9 1/2 hours
GoW2 took 12 hours
GoW3 about 10 1/2 hours
They got slightly easier with each successor imo, this is mostly because of gameplay improvements from game to game which made them easier to play. All are a good challenge on the hardest difficulty if that's what you're after.
