Light is very important not only to grow plants but for fish. Everyone who has mentioned a set schedule is correct, and also correct in that it can be what you like, at least within reason. There are a couple of issues in this thread though, on which I would like to comment.
One is feeding; except for nocturnal fish, do not feed after the light is out. The explanation is covered in this article: http://www.tropicalfishkeeping.com/f...er-fish-81982/
The duration of the tank light for a planted tank is basically determined by the plants' needs and algae. Algae is not detrimental (generally speaking) in plant-less tanks, but in a planted tank too much light will cause it to increase and it will attach to plant leaves and slowly suffocate the leaf and probably the entire plant. Keeping the light at the point where algae does not increase is the goal, and this period is subject to the tank's biology; fish load, plants, volume, feedings, light, water parameters, temperature, snails, bacteria, substrate... all these affect the biology.
The main thing is to have a decent period of complete darkness. This is crucial for fish, and necessary for plants; both need a rest period, just as we and all animals do. This is also touched on in that article.
The "siesta" approach mentioned by a couple of members--being a period of lights on followed by a period of lights off and then another period of lights on during the day--is used by some planted tank aquarists. I have been unable to find any reliable scientific data on this, and I am concerned that it may not be in the best interests of fish. All living creatures operate on a biological clock that regulates many aspects of our systems. Light is the controlling factor. You suffer jet lag solely because the change in the 24-hour light/dark period changes and your biology needs time to change or adjust. This occurs in fish too, even moreso than in humans. Cell receptors capture light more than the eyes. I have been dealing with a cancer for the past five years, and one thing I learned is that complete darkness is necessary so that the cells in our body can properly function, and many maintain this is a significant way to reduce the risk of cancer--getting nine hours of sleep in a totally dark space. I caution the "siesta" method because it is interfering with the natural biological clock inherent in fish. I've no scientific data either way, but it does seem a valid concern.