Depends on what I'm trying to grow, the more light you can give them, the more nutrients they can consume. Although I rarely grow any Caulerpa species anymore, most don't need a rest period, so can be grown in 24 hours of light. Most other commonly used macros want a rest period from what I've seen. I'm not sure of any studies, but in my experience, 6-8 hours of darkness is beneficial, i.e. they grow better. Any more than that, and you are limiting photosynthesis. Lucky for us, under high lighting, corals only need 6-8 hours of light, so lighting your refuge opposite the display's main lights works great. Lets not forget the pH balance you will recieve from doing this too.
So the short answer, 18 hours of light ran opposite the display.