Do cold cathode fluorescent lights (CCFLs) degrade as a function of idle time?
Probably the ~10k hours of actual use. According to this article in Appliance Design a typical CCFL at room temperature will be down more than 20% after 10K hours at room temperature. Of course designs will vary in their lifetime, but your 25% drop does not appear to be far out of line. 

 

Interestingly, it appears that even a short time operating at very low temperatures can greatly reduce the lifetime for CCFL lamps.