The Waterpebble sits next to the drain and uses a series of lights to alert the user when to start rapping it up. The yellow light means you should be half way done, and the red light means turn off the water. It even shaves a bit of time off each shower to help train the user to get going faster with each use. It got 4.1 out of 5 by 18 reviewers at Uncommon Goods, so hopefully that means it's more effective than my current method of banging on the bathroom door and yelling.
Waterpebble, available at Uncommon Goods, $10