What exactly stops the long killer process?

The long killer process stops its operation when the task or condition it was started for is completed. Usually, this happens when all the 'long-lived' processes it was supposed to kill have finished, or when the process itself receives a termination signal (e.g., SIGTERM or SIGKILL). Thus, the long killer's work ends either upon reaching its goal (killing long processes) or due to an external stop signal.