IIRC the last time this came up, the expected operating life of the RPF screen is 20K hours (which--in theory--would be about 3 years of 24/7 time). In practice, it could vary quite a bit. I've been running one about that long with no sign of failure as my "alarm clock". About two-thirds of the time, the backlight setting is 200 (of 255), and about the other one-third is a setting of 16 of 255. There are short intervals with a setting of 50 of 255. The lower backlight setting may very well extend the MTTF. Ask me again in two or three years to find out if I've had to replace it due to failure.
All bets are off if the RPF ever brings out a 10" screen, as I'll upgrade and "start the clock" all over again.
I have RPF screen I bought during the first week of release. It is still working just fine, but it's only used intermittently so it doesn't have very much powered on hours on it.