Plants grown from seed only ever on 12/12, never have ANY time to put on any substantial, or even semi-substantial size, and I would recommend AT LEAST a month under 18/6, before you go to 12/12, but in my opinion, Ive seen the best results when plants are left under 18/6, and vegged out "properly", until they become sexually mature, showing their preflowers, BEFORE switching to 12/12, because there's no "forced jump" in their growth, there's proper "seasons" and allow for proper formations, and strength and hybrid vigor to be in place before "throwing them" right into flowering... Kindof like expecting a toddler to go from daycare to a full-time job before they have even gone thru high-school, u know?