Assassin's Creed Mirage will take 25-30 hours to complete – and that's if you're a completionist

During what I still consider the golden age of gaming, which is 1997-2004, most single player games were aiming for 30-35 hours. That has been my sweet spot ever since but it doesn't mean a game can't be satisfying with less than that.

I personally don't find anything shorter than 10 hours enough of an experience. 25-30 sounds very reasonable.