The length is not the issue. The ratio of:
the amount of time it takes to complete quality content/total amount of time the game takes to "finish"
This!
I've played amazing 60 hour rpgs packed full of quality content that I didn't want to end. On the other hand, I've also played 20 hour rpgs that felt padded out and I was forcing myself to complete.
The time taken to complete a game isn't a solid indicator of value or quality content. Well, with an exception that it is really short. I expect a AAA rpg to last over 20 hours.