You can start your photo period on a few hrs per day with your MH's and then over the span of a few wks up your time from say 3-4 hrs to 7-9hrs.
In regards to the duration of your photo period, this is something that is tank specific IMO. It depends on the corals and your supplemental lighting. Me personally, I run my MH's for 6-7hrs a day and that is it. If your corals start to fade and look bleached move them down and visa versa. BTW metal halides will do wonders for your corals, congrads on your upgrade.