I've never really cared about the length of a game. I grew up with the original NES, so I always thought of games as something you picked up and played for half an hour at a time. Every once in awhile you'd come across a game that you could play for a couple hours at a time, but usually that was because you had your own fun with the game. For example, I could sit down and play NHL 96 on my Sega Gen for several hours at a time because I played games back to back to back. But I never sat down and played Mario 3 on the NES for more than 1 hour at a time.
I don't really think games have gotten longer or shorter as time went on. I think they got longer as available memory increased and the "save system" replaced passwords and the like. But even then you still get a mish mash of games, some longer than others. I think the nature of the industry and how we play games has changed, especially as rental stores emerged and developed. We now associate quantity with a game's value. It's not enough for a game to entertain us, it must be entertaining for a minimum number of hours. If it can be beaten in a week or less, then it seems unworthy of a purchase, because most rental stores will give you the game for a week. A game isn't simply worth playing or not worth playing anymore, it's judged on a whole different scale of criteria: how long is it worth playing for? Are there incentives to play through it more than once? Is their multiplayer? Is there online? Is their accomplishments/reward systems?
We look at games very differently than we used to, and we expect a lot more from a game. We also see playing a game without direct objectives, and simply "for the fun of it," as a bit of a waste of time. A game like Assassin's Creed can be short if you simply jump through the hoops the developers placed for you, but like many older games I played as a child, doing what the developers wanted you to do and doing what you wanted to do in the game were two very separate things. I think people today want to feel that the time they spend playing videogames is somehow productive, as though they are accomplishing something by playing it. To this end I think people are disappointed when they feel they work astutely to that end and reach it very quickly. I think if people played games for sheer fun, like they used to, then the length of a game would not be an issue, as it was not before.
