optimal length is entirely subjective and changes with other factors
WoW, esentially endless (if you like that sort of thing) but it should be because you pay a subscription etc...
portal...lasts less than 4 hours, but the amount of gameplay and fun packed into those hours is insane, so its well worth the money (which was cheaper)
I played dark messiah, might and magic, and that game lasted me about 15 hours...but it was the sort of game that should have been finished in 7, therefore it was far too long for me
If you ask me, those popcorn games you get for ps3 and play over the weekend, like uncharted, they should be around 8 hours, finish em over the weekend and move on
but games like fallout, or other rpg's anything above 50hrs is enough for me