I tackle this by setting the sleep timer in my app of choice, Listen, to fifteen minutes. As the counter winds down it plays a few low-pitched beeps over the last 30 seconds of play and if it detects any movement during that time it resets the timer for another fifteen minutes. It's easy enough to work out where you're up to in a small range.

It also has bookmarks but I never use them.

https://play.google.com/store/apps/details?id=com.acmeandroi...