Does the screen look dim when it is working (at full brightness)?
Does the light source appear to flicker a bit and flicker just before going out?
I assume you checked your energy settings and reset the PRAM/NVRAM or SMC as needed depending on your model. Here's links to Apples TN's [http://support.apple.com/kb/HT1379|Resetting PRAM & NVRAM] & [http://support.apple.com/kb/HT3964?viewlocale=en_US&locale=en_US|Resetting the SMC].
Also, does your clock setting appear to get lost? when your NOT connected to either a wired or WiFi network when you first start up your system. If so, your PRAM battery needs replacement.