From what I can tell from some reading on the internet, increased brightness decreases the longevity of LCD displays, so logically a very dim screen like the glance screen should be less taxing on the display. Anyway, the determining factor of the lifetime of a LCD display is the backlight, and the longevity quoted for LED TV displays tend to be between 50.000 and 100.000 hours untill you are at 50% of the brightness.
If we assume the lower number and keep the screen on for 24/7, this would net you close to 6 years of constant use of the screen, discounting the fact that the screen is turned off when it's in your pocket for example. Obviously the battery is a by far more limiting factor when it comes to your Lumia 920:s longevity and pretty much everyone will exchange it before the screen is the limiting factor.