We are honestly not sure how long it is and we try not to think about it. Sure, we want to have a game that lasts a while, but our main concerns is for the experience to unfold in a certain way and to make sure that what is in there is as good, meaningful and varied as possible. To have a certain game length as a goal goes against our design philosophy.
I understand you all are interested in knowing the game length though
Once we start doing tests of the full game we will probably be able to get some average completion time that we can share!