An RPG Should be short
This started out as a quick response to a question posed by the Rampant Coyote. He talks about how the lengths of games have changed over the years, and asks readers how long they like their RPGs. The response got a little long, so I decided that rather than posting a wall of text in his comments, and would post it here, and link to it.
This question actually has two answers. If it's a wide open, sandbox style game, like Oblivion, I say throw as much stuff into it as you can, make it just as long and involved as possible. Because I didn't play Oblivion for the story, I played it to be immersed in a world, and the more world there is to be immersed in the better. However, for story based RPGs, I prefer shorter games, and here's why:
First, I get tired of the settings. I want to play something post apocolyptic, then I want to play high fantasy, then I want to play sci-fi, by 100 hours I'm sick of whatever setting I've been playing and want to do something new. At about ten to twenty hours, a game is just long enough to give you a good chance to explore the setting, have fun with it, and be done before you have a chance to get sick of it.
Second, I get tired of the mechanics. You can have plenty of growth over a hundred hour game, but you're still doing fundamentally the same things at level 100 as you are at level 1. Even if they are in the same genre, there will be enough difference between two games to make it fresh. I also like the feeling of learning and exploring new mechanics, typically you don't get new mechanics late in the game, they are just iterations of the same old ones.
Third, it takes months, or years, to play through a hundred hour game, by the time I get to the end I've forgotten everything that happened at the start. I can't remember the events that so cleverly foreshadowed what happens at the end, nor can I remember the questions that are finally being answered by the big bad's final monologue.
Finally, if a game is really great, I want to experience it again, that's really hard with a hundred hour game. I recently replayed Metal Gear Solid in one day, you couldn't do that with a game that takes a hundred hours to get through. Additionally, I like exploring different possibilities, especially in RPGs. If the game is really great, and has a lot of options, I might want to go through as a rogue, then as a warrior, then as a diplomat, just to explore the different possibilities. If the game only takes ten to fifteen hours per play through I can easily do that, if it takes a hundred hours, it aint gonna happen.
One really good example of this last point is Deus Ex. That game had so many possibilities for how to play, how to build your character, and what branches in the story to take. I would have loved to play that through two or three times, but I've only been able to get through it once. I started playing through again almost a year ago, and got half way through before another game pulled me away, but I feel like I missed out on a lot of what that game had to offer.
Last year I wrote a post on how I thought that the optimal game should be around eight to twelve hours. I still think that the best game would be one that you can beat relatively quickly, but has so many different ways to play that you keep wanting to come back for more.