I realize this question was asked awhile ago, but to answer: the brush head replacement indication on the Mia 1 and Mia 2 is as follows:
After 180 minutes of usuage a decending tone will play and the lights surrounding the power button will illuminate to indicate it's time to change the brush head. To reset the brush head indicator, press the power button for 3 seconds. An ascending tone will play and the lights surrounding the power button will illuminate to indicate the reset has occured.
This was included on the instructions that came with my Mia 2, but not on the directions on the Clarisonic web site.
Hope this helps 