My first ever playthrough of Thief Gold, according to Steam, was about 43 hours... And I wouldn't hav it any other way.
On the other hand, The Longest Journey also lived up to its name and it annoyed me to great lengths with its fetch quests. I think longevity isn't an important factor, but rather the content and the variety in its content that it shows you along the way. For example, I loved Thief Gold despite the length, because every 2-3 levels it introduced something new that made you re-evaluate your approach on things. The Longest Journey on the other hand stayed a standard point-and-click game all the way through, and even though the story was colorful and had a lot of various and diverse situations and environments, I couldn't help but feel that it was getting tedious.
And don't even get me started about long games that you're not able to enjoy, like Final Fantasy 13 for me. I'm happy for all the people who did enjoy it, but damn, that game was no cool for me.