Here is something from a fish tank guide that may help you some.
If you are only keeping fish in your aquariums, you only need to replace your light bulbs when they burn out, and this varies from bulb to bulb. If you are using N.O. fluorescent lights, you should replace the starter each time a bulb burns out, as often a burnt out bulb burns out the starter as well. If you are raising live plants or corals, you should replace your N.O. fluorescent lights every 5-7 months, because the intensity of the light begins to fade and it looses its helpfulness to the plants or animals that need it. You may not see this difference until you replace the bulb, but it certainly makes a difference to the plants or animals that depend on it! If you are doing this, remember to replace the starter(s) at least once a year. If you are using power compact or V.H.O. fluorescent lights, you should replace the bulbs about once a year, or whenever the bulb burns out. These bulbs are better able to maintain their spectrum over time, and do not degrade as fast as N.O. fluorescent bulbs. Metal halide bulbs should be replaced every 8-12 months. When changing M.H. or V.H.O. bulbs, remember to check the ballasts to see that they are working well and are not overheating or showing any other signs of wear. If there are signs of wear or other problems with the ballasts, they should be replaced at the same time.