That depends on if you have been cloning or growing from seed. The clones are the age of the mother plant and can go to 12/12 after developing roots. If you are growing from seed it is a good idea to wait until the nodes on the plant start alternating in order to be sure the plants are mature enough to develop. Some people go right to 12/12 as soon as possible. A longer veg state will insure a better yield. It also depends on if you have height restrictions. Good luck with your grow!