I stop playing when it stops being fun. Are you having fun, or are you slogging through for the achievements? Sometimes achievements breathe new life into a game, making it challenging to try some new play style. I remember spending a lot of time on the Burnout crash challenges trying to squeeze more value out of each crash to unlock cool trucks. Eventually I realized I wasn’t enjoying the crashes, I was counting frames to try to time precise collisions based on cheater’s guides, and I’m like “What the fuck am I doing this for?” It was fun, and then it wasn’t. That’s the time to stop.