the "correct" length of the game an rpg is subjective. thus developers try to make it so that you can rush it in a few hours (15 or so) but if you try to savor it for the fact of savoring it you can stretch that to close to 40 hours.
For me the "right" amount of game play is the amount that is required to keep the story moving at a good clip but not loosing anything to "OMG we just learned a GIGANTIC plot twist because we took the wrong turn at that intersection!" moments.
That's the one reason I don't like most of the games that are being rated well right now. Grand theft auto and most other games that come after it are little more than glorified sandboxes. It's gotten quite pervasive in that most adventureish games try to take after it (spiderman 2, ultimate spiderman, hulk, True Crimes, Driver, Godfather...)
so for me it's "and the point of doing this quest/objective is what (other than obtaining doohickey number 7)?"