The average player will probably take about 8-10 hours to play through the game the first time. Assuming you liked it, you will probably replay it at least once or twice to try a different approach and find things you missed. The most obvious reason is to play stealthily if you were trigger-happy before, or vice versa. The former is surprisingly involved, but can make for a rewarding experience if you manage to make it all the way through an area having avoided detection. Doing so is rather tricky because of the checkpoint save system; it would've been nice to be able to quicksave.