Depends on the game, and WHY I'm playing.
I played WoW for 4 years, and only had maybe 100 Hours in it that I've had actual fun. (Being and MMO 100 hours is nothing), the rest was grinding and killing time til I needed to do something or a raid occurred that I was going to. I kept playing because while the game was getting tedious, talking with my friends who played it, and their shenanigans, as well as talking to the rest of my guild (nearly 200 people, across like 1500 toons or something.) was fun. I've found myself logging on, and sitting in Dalaran or Stormwind and just talking to my guild, while on skype with my real friends.
I quit that because I was getting bored, and both my IRL friends, and guild friends were quitting.
I've played through Army of Two, twice and it's a boring game, even the first time. However the two people I played through with made the game much more than it is by itself. Luckily the 2nd on was much better, even if it isn't the best game out there.
I played Far Cry 3 for 4 hours and just quit... Came back last night and finished it. I think I bought it Christmas 2012.
90% of my Steam games I've never played, because I got them in $5-$10 bundles (Thanks Humble Bundle).
It's usually when I stop having fun with a game.