It depends on the type of lighting, the needs of the tank, and the nutrient load.
If the tank can handle it (the nutrients are not too high), running the lights longer will allow more coral growth. Some people run them up to 12 hours of white, with an extra hour of actinic on each end to simulate dawn and dusk. In many tanks this will promote extra algae and the white (and therefore the actinics) need to be cut back to ten, eight, or even six hours. Actinics should always run an extra hour on each end. Moonlights can stay on 24 hours, or you can turn them off when the other lights are on.