The duration of light your garden requires, depends on what cycle the plants are in: The 'Vegetative Cycle'- of your garden begins with the sprout of the seedlings. In the veg cycle your garden will require a minimum of 16-18 hours of light per day. Since a plant can be kept in the 'Veg cycle' for a very long time, many growers cultivate 'Mother' plants. The plant is then used for cloning and never produces buds, only new growth. The 'Flower Cycle'- or 'Bud cycle' is typically12 hours on, 12 hours off or 12/12. This produces a change in the plants growth simulating fall, shorter days, less light. This is when the plants will show their sex. Usually, you'll be able to determine the sex within the first 2-3 weeks of 12/12. By the 3rd week most plants have developed healthy bud sites or pollen sacks. Continue on the 12/12 cycle until harvest. 'Re-vegging'- Growers are able to harvest plants more than once with most strains. This is done by harvesting all the bud at the end of the flowering cycle, except the bottom buds. You then keep the plant in a 24/0 light cycle (24 hours of light per day). In as little as a few weeks, the plant will switch back to the vegatative state, and develop new growth (shoots). When the plant has enough new shoots, you can put it back into the flowering cycle, to harvest another crop.