Anything under 40 will always be short. Good stories need time to be told. So unless you go around skipping through the dialogue 40 is a good core time.
The best games give you a good story and plenty of other activities (which contribute to it).
Funny thing is. It's only since the former PC developers have gotten in on the act that console RPGs have become short. Appart from the odd exception they tend to average 60 hours unless you rush them. Find all the extra's and secrets and you can double that a lot of the time.
I'm not sure I would count Oblivion, since yes its big. But there isnt much variety or any real point to the exploration. So 500 hours of wandering for no purpose ,not my idea of fun.