how long to run metal halide

I have one 175 watt MH over my 40 breeder. How long should I run it for. Right now I have it set for about 7-8 hours.
 
right now i would start out real slow maybe 3 hours and slowly raise the time limit...depending on the reflector you can go as low as 6 hours but 7 to 8 hours is the norm
 
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