Depends on water depth, clarity, coral type (different acropora take better to different light, etc) I have had the same lighting over a few tanks and all of them had different photo-periods, I have also changed photo-periods due to changeing bulb-temp/color.
If you give more info on the specific system (tank dimensions, parameters, livestock, etc) you are refering too it would be helpful and would get you more accurate answers.
--John