Was doing some reading in a garden book.They are statting that 18 hrs of light the best for growing as allows the plant durring the no light cycle to put energy into growing the roots. Bigger root system more the plant can support the topp plant and bigger storage area for when you go to the flowering stage. I am new to this and have found the size of the room a bigger facter in the hieght of the plant therefor the time in growth. Also from what I read the higher the plant more energy the plant has to use to bring the food to all parts of the plant and less food for the flowering.