Your UV bulb should be replaced each year in the Spring. We recommend around Easter. The bulb will typically be effective for 8000 hours, so by changing it in the Spring it will be producing the optimum output through the brighter Summer months.
We also recommend cleaning the quartz sleeve when you are changing the bulb. The bulb is inside the quartz sleeve. The sleeve can become dirty and calcified reducing the efficiency of the unit. Take care when cleaning the sleeve as it can be easily broken.