Metal Halide bulbs can last for several years. However, it is suggested you change the bulbs every 12 to 18 months. This is due to the spectrum output of the bulbs changing from blue to yellow as time passes by.
Don't wait until your light bulbs burn out, or you can see the change in the color spectrum. Set a schedule for changing your bulbs.
Note: This bulb requires an HQI or electronic ballast to operate effectively.