Whenever I feel done with it. I played sixteen hours of Dark Souls, fought about five bosses and got all I ever want out of that game. I have all of the achievements for Oblivion but so long as I can still install wacky mods and stumble into new dungeons, I'm never going to be finished with it.
Oh the illusive middle ground. Where art thou?
Seriously people, learn to make polls that aren't fucking black and white. It's not hard.
As for my answer, merely playing a game to the credits, especially nowadays when games are so short, isn't usually enough to satisfy me. But I also rarely have any achievement or trophy based aspirations. To me, I feel I have completed game when I feel satisfied by it. In other words, when I've had my fill of its gameplay and it's no longer stimulating or fun. This can be achieved through multiple playthroughs, conquering harder difficulties, or digging into multiplayer.
Like other have said. Depends on the game, it just needs to tickle my brain in such away. MG Rising tickled it enough to complete it 4 times, 3 times in the one day.
While Far cry 3 I am pretty ready to pack it in and I just got the wingsuit. Going to complete the main story and then trade it in.
Fallout 3/Oblivion/Skyrim I do not consider complete despite finishing the main mission.
Sleeping dog I completed yet I dont feel its completed....I am not changing achievements around either.
When I've done everything that doesn't feel like an extra optional thing. Smash Bros. Brawl had credits but you've hardly completed it by the time you see them. You're also probably not going to collect and unlock everything extra just to say you've completed it.
I'll play until I'm not having fun, but I hate leaving games unfinished if I can help it. I at least have to see the credits roll. Cheevos forget it..I play lots of RPGs, open-world action adventures, etc, and there's too many esoteric trophies, MP trophies I'll never get, or ridiculous collectible hunts I'll never finish. I like achievements too. If I can finish a game's main story and get 65% of the trophies, I consider the game done and dusted.