I work on MacBook Pro (15", mid 2010) pretty much every day for at least 8hrs. I remember my previous (non-Apple) laptops displays used to darken over time. Should I expect something similar happening to my MacBook Pro?
I have had a MacBook Pro since early 2007, and the display is as bright and crisp as the day I bought it. I've also had an iBook G4 since early 2005 and there's no problem with its display, either.
I doubt you'll have any problems for quite awhile if at all, as newer models tend to have better displays then older models.
But by how much?
There are plenty of display technologies out there (LCD, LED-backlight, OLED, ...) which makes a general answer quite difficult. Most of us will also recall some pretty fancy abbreviations like TFT, WXGA...
I'll radically simplify the problem by assuming (!) that we may distinguish between only two basic display technologies used in Apple's Macbook Pro. Each technology has experienced little quality improvements over time, but distinction between the fundamental technology behind should be more important:
I do not have official numbers regarding the lifespan and change in lux of Apple's displays, so I'll tell you my experiences:
On my Macbook Pro mid 2007 I've had to use a higher brightness level year-by-year. This is not as bad as with the LCD display of my black macbook, but degradation is noticable! Today, after more than four years of almost daily use, the black Macbook's brightness is not acceptable in summer. The MBP does a fine job, but during the day I constantly use one of the top four brightness levels.