If you think of it closely, burn in is impossible with eink. There is'nt enough current in the process of displaying that can alter the physical aspects of all materials involved.
The only processes that can degrade the perfomance of eink are:
Lack of power. This is responsible for most ghosting. I'd like to see an option command to incite a black refresh screen.
Evaporation or thickening of particle carrying liquids.
Photosensitive chemical changes in the two colors of ink( I'd suspect mainly the white one)maybe brought by UV sensitivity.
Theoretically, eink will last a long time. That's the genius of it.