Useful life of HID bulbs is pretty short. I'd change your bulb annually. If you got it used with the bulb you are still running, it is almost certainly time for a new one. Lumen output drops while energy consumption stays the same, and your efficiency goes way down.
Just as a safety precaution, I think it is generally good practice to swap out bulbs well before they burn out on their own. I've heard of HIB bulbs actually breaking when they burn out, however few growers will ever see that because their useful life to grow plants is much shorter than the time it takes them to wear out entirely.