They need more than 15 hours of light a day during the vegetative stage, otherwise they may start flowering. Give them at least 18 hours a day during vegetative growth. Then when you want them to flower reduce the light to 12 hours per day, and 12 hours darkness. Remember, when flowering the dark period must be DARK, no light whatsoever. This is very important. If light gets in during the dark period it will disrupt the plants and mess them up.