not positive but the LCDs themselves hold up rather well, it's the backlight that is the "weak link"
i think they are rated for 3 years of use at best performance and after that its a crapshoot. I have had my powerbook g4 since fall 2001 and the screen is fine, may be slightly less bright now though \
There were a few PB models that had notoriously flakey backlights. I haven't heard of any problems recently. As long as you don't have the brightness cranked up constantly, the screen should last.
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i think they are rated for 3 years of use at best performance and after that its a crapshoot. I have had my powerbook g4 since fall 2001 and the screen is fine, may be slightly less bright now though \