Well LCD screens should on powermacs should llast around 20,000 hours.
Have you checked all your power manager settings?
If its no longer in warranty and you've tried everything you can you may need a new screen, they are fairly simple to replace but if you don't feel confident (or you are likely to break it) there are plenty of places that will do it that are not as expensive as the apple store.
Natalie
December 2008