E Ink film is sensitive to UV light. It will optically degrade without proper UV protection. We've had a display without protection on a sunny desk by an open window (so no UV protection on the windows as well) and we got permanent burn in after a couple of months - this is why all finished devices take extra steps to protect against UV.
Properly protected screen will last for ever - we have units in the office running for at least 5 years without stopping.
