You can find out a plant's critical light level by observing plants outdoors to see when they start flowering. Check the length of the darkness period when you observe the first flowers. Indoors, you can start at 9 hours of darkness and increase the dark period by 15 minutes each week. Figure the critical period as 15 minutes shorter than the dark period length when you see the first flowers. Suppose a plant's critical dark period is 10 hours. This means that it will flower under 14 hours of light per day. This is 2 hours longer than the 12 hour light period which is generally recommended. The two hours extra light each day is a 16 percent increase in light to the plant. This is critical because light = growth.
A shorter dark period may lengthen the flowering period a bit. To get the buds to ripen quickly, the dark period can be lengthened to 12 or even 14 hours.
