An HID bulb can lose up to 30% of its efficiency over two years; up to 10% after the first year. The spectrum can also change over time, due to the intensity change. A frugal grower might replace their bulbs every two years. A commercial grower will change the bulbs annually. The (type A) overly conscious grower might replace them every 6 months. (Fluorescent, LED, and Plasma lights, do not apply- these bulbs are good until they burn out.) If you buy a light meter: When you get a new bulb check the lumen output and write it on the box. Then you can take a reading every 6 months to gauge when to replace the bulbs for newer, more efficient ones.