I like most games to be under 10 hours, but with RPG's the sweet spot is between 20 and 30. That way I can get really invested but the game won't overstay its welcome.
I've played some amazing 50 hour RPG's when I was younger, but it's just hard to do these days. I absolutely love The Witcher 3, for example, but after 30 or so hours I fell off and haven't gone back. Some day, I keep telling myself.