I know that over time the power spectrum of a fluorescent light changes. You start to lose the shorter wavelengths (UV, blue) as the phosphors wear out. I also know that plants need those shorter wavelengths to be happy in an aquarium. So, my question is, how often should I replace the fluorescent tube in my aquarium to keep my aquatic plants happy?

This guy seems to make sense:
"Finally, if you have the money, buy a timer to turn the lights on and off at regular 12 hour intervals, so the plants won't get confused if you're not home to turn the lights on and off at the correct time, or are on vacation. These run about $10 at our local hardware store. If you have a small or odd sized tank, the hood that came with the tank will satisfy neither (1) nor (3) above. That's ok, just buy plants that do not require a lot of light. As a general rule, dark green plants are ok with low light levels, and light green or red plants need high light. If you can afford it, add a second light fixture to the aquarium so that there are two light strips above. You will notice a marked improvement in growth. I have modified some fixtures to fit three 15 watt bulbs over my ten gallon tank, although the plants could get by with two. The intensity of flourescent bulbs goes down dramatically in the first 6 months of use, so having a lot of wattage means that you don't have to worry about replacing the bulbs every 6 months."

It's from here. Check out the entire LIGHTS section there for some good tips.