This depends on a number of things. 1) Do you have live plants? 2) Are you just interested in seeing the colors of your fish? 3) How often and how long do you use your lights? 4) Is the color temperature important to you?
Fluorescent lights do change their properties over time, but if you are only interested in seeing your fish, just wait 'til they burn out. You can tell if lights are getting old by examining the ends of the tubes. Darkening indicates that they are getting old and might not be performing up-to-par. At this point, you might not be getting the full spectrum of light needed for plants. If you need a particular color temperature, I would recommend purchasing a color temperature meter.