I'll just offer a few comments on this question of light duration.
First, fish do not need overhead light at all. In a tank with just fish (no live plants), the light is solely there for our viewing, so the schedule can be adjusted accordingly. However, a period of total darkness is essential, and I'll come to this further on. Ambient room light is fine for the fish; many of the forest fish live in this environment all their lives.
Plants require sufficient light to photosynthesize, and photosynthesis is their method of growing. Without light that is sufficient in intensity and duration, they cannot live. So light of a minimum intensity (depending upon the plant species, some need more than others) is essential. But the duration is governed by the availability of nutrients, and duration can be as little as six hours.
Back to that darkness: plants and fish need a period of complete darkness within each 24-hour period. This is their rest, and it is essential for a healthy life for both fish and plants, just as it is for us. I go into this more in the article on light: http://www.tropicalfishkeeping.com/f...er-fish-81982/
In the tropics from where most of our fish occur naturally, light and dark is about even. Ten hours of total darkness and ten hours of daylight, with dawn and dusk being the remaining four hours. "Daylight" is not usually full sunlight. The forest canopy shades the streams; floating plants cover watercourse that receive direct sun and it can be quite dim under these. This is also expanded upon in that article.
The duration of sufficient light is essential to plants, and it is no surprise that many tropical streams have no living aquatic plants, simply because there is insufficient light. But in the aquarium we want plants to thrive, so we provide light. But it must be balanced with nutrients. Plants can only photosynthesize if everything they need is present: sufficient intensity of light plus all 17 nutrients. As soon as one of these is no longer available, photosynthesis slows down and may even cease completely. We call this the limiting factor to plant growth. Light should always be that limiting factor; if light continues and nutrients are unavailable, plants slow or cease using the light and algae takes advantage. So we limit the light solely to control algae; algae is therefore a useful monitor on the light duration.
To put all this together. Too much light, whether intensity and/or duration, that is not balanced will cause algae to increase. A period of complete darkness is also essential, say 8 or more hours. More will not hurt, less will. So the tank light can be on for any period from 6 hours to 16 hours, approximately. Arrange the light for when you are normally there to enjoy the tank, and a timer is best for this as a regular consistent period of light/dark is better for both plants and fish.