most people have anywhere from 4 to 12 hours a day of MH, on the lower end they are usually supplemented with T5's. I think 8 to 10 would be a good average, and I'm actually using mine seven hours a day currently with good results.
what I believe oppih was saying is that if you are talking about symbiotic algae which is what corals live off of, start with your lights for very few hours a day as the MH's are probably stronger than the holding tank at the LFS's are. Even if not, it helps them acclimate without overwhelming them. A lot of people that have T5's and MH's will set some actinic (blue) T5's to come on first to simulate morning light, then later on, say two hours, the MH lamps come on. After the MH's have been on say eight hours, they go off but the T5 actinics stay on for two more hours. That's an average photoperiod, but everybody isn't the same. I personally have my tank in the living room near a large window, and it gets a lot of light normally during the day, and I do not use T5's, just MH only. So my lights come on at 2 PM, and go off at 9 PM. You want the total photoperiod to be around 10-12 hours a day, and that doesn't have to be full MH the whole time, it can be supplemental with something else, and that's what there is such a large range. It completely depends on what you intend to keep, if you are using any other lighting, etc... On the other hand, if you are trying to grow macroalgae, you don't necessarily need MH's to do that, they will grow with almost any light, and 8-12 hours a day will work just fine.