During the propagation stage, lights should be left on as long as possible, up to 24 hours per day. Strong initial growth will make the young plants healthier and more resistant to disease, ultimately providing better yields. As the plants begin to mature, however, light duration should be reduced.
Plants use periods of darkness, too. During dark periods, plants continue to take in oxygen and give off carbon dioxide in a process called respiration. During this time, the plant uses some of the energy that was stored as sugars during photosynthesis, producing proteins, hormones and other complex products. Therefore, lighting mature plants more than seventeen and a half hours per day will not increase production significantly, since photosynthesis will slow down after a certain number of hours. I have been using this method or years and see great results. 18\6 is a easy number to remember.