A great example would be Dragon Quest IX, Sentinels of the starry skies for the DS. I've played it for like 6 hours now, and I don't even have any party members and I've been to only one city. The game is SO immense, but also SO awesome.
But Dragon Quest is of an insane quality. Everything seems rigt, the graphics are totally awesome, there's a little bit of retro in it, nice music, beautiful animated cutscenes, great variaty in monsters, a lot of sidequests, one big story with smaller stories in it... I couldn't name it all.
What I'm trying to say is that your game has to have something really awesome in order for me to really play it that long. The fact is with RM games that they get checked on how well you made all that stuff rather than really playing it.
Making a good RPG is really hard. Me myself, I'm usually not very interested in RPG's, but Dragon Quest instantly got my attention and it didn't let go. So if your game's just an average RPG, I think I wouldn't go for the 40 hours, but if it had something that would REALLY attract me, like Dragon Quest has (which I'm about to find out what that specific thing is), I would defenitely play. The hardest thing to do is just creating something that makes people continue playing your whole game.