Eric,
Exactly what you described is what I was concerned about. When flying one of the patterns you know when its over and have several laps to spare. Sport flying has you just cutting up the sky with say a combat wing or biplane and just having fun but not watching time. I had a cook timer with alarm but it is a pain to carry in the summer with just a tee shirt (no pocket). Was looking for something small but loud to warn you.
Best, DennisT