Most music players have the option of a sleep timer:

An implementation by an app sounds pretty straight-forward to me, where calling stop() or play() would stop or pause the player, which then would take the service out of the foreground which makes the system terminate the service after a given time.

There isn't a way to let the player pause at a given time by API. There are APIs though that can help you do that.

1. You can calculate from what you put into the playlist and the do

    player.createMessage((messageType, payload)
            -> player.pause())
        .setDeleteAfterDelivery(true)
        .setLooper(player.getApplicationLooper())
        .setPosition(player.getCurrentMediaItemIndex(), positionAtWhichToPause)
        .send();

to pause at the given position.

2. You may don't know the durations of the items you put into the playlist ahead of time or the user may manipulate this after the timer is set. So you can add an listener that is called each time when the media items changes. I assume your app provides a method getSleepTimerEndMs() that gives us the time in millisecond, at which the sleep timer ends:

@Override
public void onMediaItemTransition(@Nullable MediaItem mediaItem, int reason) {
  if (reason != Player.MEDIA_ITEM_TRANSITION_REASON_AUTO) {
    return;
  }
  long nowMs = SystemClock.currentThreadTimeMillis();
  long timeAtWhichCurrentItemEnds = nowMs   player.getDuration();
  long timeAtWhichTimerEnds = getSleepTimerEndTimeMs(); // app knows whether timer is on
  if (timeAtWhichTimerEnds != C.TIME_UNSET && timeAtWhichCurrentItemEnds > timeAtWhichTimerEnds) {
    long positionAtWhichToPause = timeAtWhichTimerEnds - nowMs;
    player
        .createMessage((messageType, payload) -> player.pause())
        .setDeleteAfterDelivery(true)
        .setPosition(player.getCurrentMediaItemIndex(), positionAtWhichToPause)
        .setLooper(player.getApplicationLooper())
        .send();
  }
}

You may want to monitor other events, that cancel, send or update the message in case the user manipulates the player during 'sleeping'.

Keep your message in case the user changes or cancels the timer. The you can call message.cancel().

You can use CountDownTimer and a handler to handle the scenario. I have implemented using the same with 30, 45, 60 mins and an End of Episode sleep timer. When the timer runs out, an onTimerCompleted is called if you use CountDownTimer. Inside the onTimerCompleted, I am simply doing controller.pause() or cotroller.stop() depending use case.