How varying amounts of light per 24 hour period affects plant growth.
The amount of time your garden should be exposed to light depends on what cycle your garden is in:
The Vegetative Cycle of your garden starts with the sprout of the seedlings and can be continued indefinitely. In the veg cycle your garden will require a minimum of 16-18 hours of light and 6-8 hours of darkness daily. Since a given amount of light can only do so much, equal production can be realized in a smaller space with less plants, where the light is concentrated and the plants can grow more efficiently. Using more light helps additional co2 uptake. Many growers cultivate a "Mother" plant by keeping it in its vegetative state indefinitely. This plant is then used for clones. It never produces buds, only new growth.
The Flowering Cycle or 'Bud cycle' is typically equal amounts of light and dark, 12 hours on, 12 hours off or 12/12. This produces a change in the plants metabolism simulating the fall season, with which comes shorter days and less light. This is the cycle that the plants will show their sex. Usually, you'll be able to determine the sex within the first 2 weeks of 12/12. By the 3rd week most plants have developed healthy bud sites or pollen sacks. The plants will continue on the 12/12 cycle until harvest.