What is the average life of LCD display?
That depends on what you define as "life" which, of course, must be twice as long when it is a LCD display (display display).
The public transportation industry still specify VFDs in many cases because the LCDs used to lose contrast to an unacceptable level well before the expiration of the device.
However, today the LCDs are coming back in acceptance and I must thus surmise that this problem has disappeared.

I know this is but anecdotal evidence, but it may in some way answer your question.

I strongly bsuggest that, when selecting a LCD, you stick to the forewfront manufacturers since the fading will not be evident initially and if it happens you will have egg on your face.

Erik