I've been using an eInk screen for over 12 years - it is refreshed multiple times per day.

It is as crisp and clear as the day I got it.

Admittedly, I'm not trying to run video on it constantly and it doesn't get hot. But eInk seems remarkably durable.

from 6.7 to 42 would be my guess.

But being serious, I personally have not seen a degraded e-ink display.