HID ( High Intensity Discharge) bulbs vary. HPS (High Pressure Sodiums) bulbs need to be replaced once a year with a 12 hours on 12 hours off cycle. HPS bulbs lose 20% output in a year, so you lose around 20% of your yield. MH (Metal Halide) bulbs need to be replaced every 8 months on a 18 hour on 6 hour off cycle. MH bulbs lose 40% output in a year, so you lose around 40% yield. Problems like the light going on & off with power outages make the life of the bulb even shorter. Bulbs will in a lot of cases work for years, but they are not putting off the light that plants need. The inside of the bulb starts off clean and as you use the bulb it starts to turn black on the ends of the inner arc tube. The older the bulb the more black is on the inner arc tube. The black does not let the beneficial light through. When a customer comes in who has been using a bulb for 2 years & they buy a new one, they always say they did not realize how much light they had lost until they plugged in the new one and that they would be replacing the bulbs more often now that they saw the difference first hand.