There are games I've played that seemed like they were too short, while others seemed to last forever. So I guess it really depends on the person playing the game.
If it's a multiplayer, with the online option, I'll play the campaign til I beat that, and go to multiplayer or play the campaign on a harder level. I actually got sick of the Black Ops campaign and jumped right into multiplayer after about half way through the campaign. Probably wasnt even that long.
I got fed up with the one chapter of Mirrors Edge, because I couldnt seem to get past the one place, so I pretty much said "Screw this" and went to a different game. Still need to go back and beat the chapter. I even looked up the chapter online to see what I was doing wrong, which was nothing, and still having problems.
So really, it's all how you play the game and how you enjoy it. If you're bored with it, take a break and come back after a while and see if you still feel the same. I still play Soul Calibur IV, but I'm sick of story mode. It's too short. Yes, every character has a different story, but you're fighting the same characters for the most part, so you kinda have an idea of what they're going to do and how to beat them. If I want to play SCIV I tell my friends to get on and play with me. I find them more of a challenge.