For me its less about actual length and more about satisfaction. The game can be 100 hours long or just 8 hours, as long as I had satisfying experience then I'm happy.
Depends so much on what's going on in my life.
When I was 20, 45 hours was cool.
Today with a job, two kids and a house, I'm struggling to play at all. So when I play I'm happy if the game doesn't exceed 10 hours.