Daylight and total darkness are equal in the tropical rainforests of SA. This equates to ten hours of "sun" and 10 hours of complete darkness; the remaining 4 hours are obviously dusk and dawn. "Sun" is a relative term, since most of the waters do not see that much direct sun but more diffused daylight.
Duration is something that has to be worked out to balance your fish and plant load and nutrient fertilization. If the light and nutrients do not balance, i.e., the light is on for longer than there are nutrients available to the plants, algae quickly takes over. In my setups I find that 10-11 hours of lights on works best; algae is naturally present but kept within bounds and the plants grow very well.
Light should always be the limiting factor in plant growth. Plants will continue to grow up to the factor (light, some nutrient) that is no longer available. If this factor is light, you will not have algae problems.
The timer can be set for any period, since you obviously want the lights on when you are normally home to view the aquarium. And you want light in the room, be it natural daylight or a lamp, when the tank lights come on and go off; this avoids a light/darkness "surprise" which is stressful to fish.
I tracked down the video of angels in their habitat that I posted a while back, here it is; the first part shows angels in a natural habitat, and you will observe they are under cover until the diver causes them to swim out. Video of angels, cardinals in Amazonia