Most locations that you can find coral reef in the world contain about 12 hours of daylight with about 4 hours of intense light. The metal halide don't really have dimming capabilities. The corals only really need these 4 hours from the Metal Halides. Then the rest of the time it could be with either VHOs or t5 lighting. Though longer times won't do any damage to the coral it'll make the light spectrum of the bulbs shift faster so you would need to change your bulb faster.
The most optimum time to change the bulb would be with the use of a light spectrum analyzer if you can and want to afford one. Otherwise the age old 6 month period works pretty good.
The lights should be turned on and off as your MH diving lights.
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Photo anyone? 
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