Your plants do their growing in the dark. The more dark they get, the more they will stretch. I've always vegged on 24 hrs. It keeps my plants compact with less space between nodes. After 2 weeks of rooting and 6 weeks of vegging, the plant only gets about a foot, maybe foot and a half tall. The same plant usually ends up about 5 to 6 feet tall and about 3 or 4 feet across. I usually get about 6 -8 ounces per plant every 8 weeks. These are blueberry plants, the mexicans are usually a little smaller.
I read in a High Times mag once that, during flowering, you can actually give the plants more than twelve hours of light, as long as they get 12 hours of dark they will keep flowering. The theory here is that the plant absorbs energy from the light and uses that energy to grow and stretch, so the extra light causes the plants to grow more in a single 12 hour period. I've never actually seen or heard any proof of this though.
