Eventually, you've learned just about all of the mechanics of the game, and you've unlocked most of the content. At this point, the game is still fun but some aspects of the game start feeling like a chore, e.g grinding in MapleStory and playing A.I in LoL to get first win of the day. You would rather a bot do it than yourself.
Finally, you will reach the stage where you are very familiar with the game, and know everything about it. You are no longer learning new things about the game, and it's not as fun anymore. You play the game to obtain something in the game, but you do not enjoy the process of obtaining it. You enjoy levelling up but not killing monsters. Once a game stops being enjoyable, that is when you should quit and do useful stuff in life or waste your life on a new game that you will enjoy.
Most League of Legends players like me have been playing the game for years and have reached the stage where most of the content has been unlocked, and we reached the max level of 30. Our only goal now is to gain LP to obtain a higher elo. To do this, we have to win ranked games. But for many people, they only enjoy gaining LP, not the game itself. They enjoy stomps or 5v4s and do not enjoy losses even if it was a good game. They get angry when someone in the team misplays because it makes winning harder.
Once you reach the stage where you enjoy winning more than playing the game, League of Legends becomes a waste of time and you should do something useful like work for $$$ or find actual hobbies (League of Legends is not a real hobby).