Subway Surfers has a clear loop: run, lose, restart. That restart button is why a quick break can stretch. The stopping point has to be chosen before the next run begins.
Use a one-run, three-run, or ten-minute rule. Set Subway Surfers inside Apple Screen Time App Limits, then track it with Sandflow so the session length stays visible.
If you play during short breaks, stop after the run that crosses your planned time. Do not start a "last run" after the timer already says the break is over.