You should Have at least 4 Nodes before flowering. Also you should go by the size of the room. A 1 foot plant at 12/12 will tripple in size before it fully matures. If you have the room and want a bigger yield wait longer and let it get to be 2 feet before you flower it. Genetics plays a part too. Preflowers are a great sign you will have a beautiful GREEN plant and the preflowers are a light green to yellow in spots. Then 12/12. Room size counts and lighting. If your lighting does not penetrate you get low yield on the lower parts of the plant.