You may already saw them on Twitter. Thanks to fablab Munich I could take some IR pictures of a running matrix.

Front at 15% brightness white:
Back at 15% brightness white:

Front at full brightness after ca. 5 sec. running... Than I unpluged it.

So it should not run a full brightness for a long time. I am thinking to use the SAMD21'S integrated Temperature Sensor to limit the Brightness to Maximum Board (/MCU) temperature - maybe to 70-80°C.