I think games are fine to be anywhere from 5 minutes to 200 hours for 1 playthrough as long as the game offers FUN & DEEP gameplay, entertaining replay value, and intrisically rewarding challeneges.
A lot of older games really excelled at these aspects of gaming, so even if I run though a good NES, SNES, ps1 game, etc. I don't really mind starting a new playthrough as soon as the credits finish.
So really, I think total playtime is a more accurate measure of value per dollar, though thats really only one factor of value.
BTW, 6 to 10 hours TOTAL play time is really, really low.