I'm not sure I follow, @lewanni. 15 hours is a very generous duration for an adventure that will cost around 20 bucks. Most adventures these days will take less that time, not to mention indie games like Firewatch or Inside lasting 3-4 hours. Like I said above, Asylum should be twice as long as Scratches, for the same price.
It obviously could be more than that, but 15 hours is the bare minimum we're expecting.