Oh, and the one I have vibrates after 30 seconds to tell you to move on to the next 'quadrant' of your mouth. After four quadrants (2 minutes) it vibrates some more to tell you to stop.

This has confused me slightly. My mouth has teeth on the left, teeth on the right and teeth at the front. It divides up pretty neatly into thirds, and rather less neatly into 'quadrants'.