Did more research and was having a lot of trouble finding 75W metal Halides (I have a centre brace in my 29 Gal tank and don't want a shadow from the brace so 2 lower wattage lights would be needed instead of a single lamp) and came across a section of the USHIO website that was quite informative:
http://www.ushio.com/products/petcare/aqualite.htm
In it, they state that MH lights should be on for at least 12 hrs a day...
No telling if that's a marketing ploy to use more lamps or if that's from a sound biological rational.