[MEtal halides will typically reduce in intensity and color shift before they fail completely.
Depending on which bulb/ballast combination you are using you would choose a timeline for replacement. It could be 6 months to 15 months or so. For example, an over driven radium might only be effective for 6 months, while a double eneded bulb on an electronic ballast might have a useful life of 12-15 months.
You can also use a LUX meter and a color temperature meter (both used in photography) to measure the new bulb and use that as a reference point. Measure again at 6 months then every two months after that till you feel the bulbs need replacing.]
