I have yet to beat the game, but devs stated that it should take about 8-10 hours to beat, depending on how much exploring, etc that you do.

Also the new game + option adds a tone of replayability. Also there are challenges to complete with each weapon, and different difficulty modifiers that add replayability as well. Some modifiers include things like enemies regenerate health, no dodging, etc.

So with all of those in mind, a person could easily squeeze over 20 hours out of the game if you tried.

I beat the game today and it took me close to 10 hours because I was trying to complete everything in my first game. I missed a few things and am looking forward to playing through again in the new game plus mode.

I beat the first run through in a little under 6 hrs (went through "who knows where" twice), doing new game plus right now and in about 2 hrs im 75% through the story mode already. The ability to keep your upgraded weapons makes things a LOT easier the second time around. the narrator makes a lot more sense the second time as well.

I've gotten a good bit of time out of this game, just because I like going through who knows where with different loadouts/handicaps.