I would say 5-6 hours to complete the game on the Casual setting is about right. It took me just over an hour to finish act 1. I think I'm going to go back and play it on the Hard Core setting now that I've got a handle on the gameplay controls.
There are also several spots where you're given a choice of paths, so there is definitely incentive to go back and replay choosing a different path to see all the game has to offer.
I have to say this game has some amazing in-game cut-scenes and a lot of jaw-dropping moments! It's incredible that they can pull off this level of graphics with a steady framerate and most importantly, no screen tearing!
PSN/XBOX Live: Destro75
BattleNet Tag: BaronessMD#1680
Gaming PC: i7 3770K OC 4.2Ghz, 16GB RAM, MSI 4G GTX 970