Indoor grow lights are terrific for gardeners who want to get a jump start on the growing season or who want to solve the problem of insufficient light during winter months. Fluorescent lights are one kind of grow light that can be used; there are a few replacement guidelines that will help you change the bulb at the right time.
Consider how many hours the light has been used. If you have used the light for the entire day, then you will need to replace the bulb sooner than if you only ran it for a few hours a day.
Each manufacturer produces its fluorescent lights with the parts that will afford it the greatest amount of profit, so parts aren't always the greatest quality. If your bulbs consistently fail before they reach the maximum number of hours listed on the manufacturer's packaging, switching manufacturers may reduce the number of times you have to replace the bulb.
There are several different types of fluorescent grow lights available. Standard bulbs will last (on average) about 25 times longer than an incandescent bulb of the same wattage. High output and compact fluorescent grow lights don't last quite as long, having a life of about 10,000 hours. Switching to a regular fluorescent grow light will result in fewer bulb changes, but regular fluorescents don't have the advantages that the other bulb types do, such as being able to fit in a small space.
See if your plant growth pattern has started to change in a way not related to the plant cycle. If your plants suddenly are reaching toward the light, or if the plants show signs of yellowing, this could indicate the intensity of the light no longer is sufficient to meet the needs of the plant and that the bulb is starting to fail even though it still is operating.
Plants can benefit from up to 16 hours of supplemental light per day when flowering (10 hours when not flowering). If you use a fluorescent grow light with a 10,000 hour life estimate and run it every day based on this standard, this means you should expect to replace your bulb at least once every two-and-a-half years. Of course, this does not take into consideration that intensity of a bulb starts to wane before the bulb is completely burned out, so you really need to replace the bulb as soon as you notice a significant difference in light output, which will vary based on the bulb and manufacturer. Replacing once a year or even every few months is not unreasonable.