In theory I would like to say that I complete game to 100% but like a lot of you, that isn't really true.
Most of the time, I just try to "finish" a game's main story. I do side quests as I feel like it but usually I do not go out of my way to do everything unless its pretty easy. But a lot of the time, this leaves a lot to be desired once you finish a game. Take Ocarina of Time. I beat it for the first time not to long ago, just main lining the story, not using a guide. Apparently I am a pretty shitty Zelda player because when I finished the game I had less than the whole top row of hearts (HP), the second biggest wallet, I missed Epona completely, and fought Gannon with a bottle and the broken giant's knife. But after finishing the game, I didn't feel bad about putting it down. I can come back to it anytime and go after the hearts, Epona, and anything else I missed. But for now the game if finished imo.
So to remedy this, I usually pick a certain collection aspect of a game to finish. Take Uncharted for example. There are little trinkets to collect throughout the game. I tandem with finishing the story, I grab all the treasures along the way to feel like I have gotten my money's worth out of a game.
But past that... not many games are really worth my time to go back and squeeze all the extra juice out of a game. I only have about a few games I would say that I have truly completed to the 100% marker. I completed FFTA and Dragon Warrior Monsters, Kirby's Knightmare in Dreamland and a few others. But even that is subjective. Saying you "Completed" pokemon? What does that mean? Beating the main story? Catching every pokemon in that generation? Catching every pokemon available in that specific version? Maxing out the pokedex? Maxing out the pokedex AND leveling everyone to 100?