depends what you want from them,i've found a 12 inch tall plant can triple in size after being switched to 12/12,i've also found that growing under 400w hps i can get 130 grams from a plant that was 12"tall when 12/12 was started,takes around 4 weeks under hps to get 12" but really no idea about outside,got a few outside myself so will have a better idea come november time,just bear in mind flowering time will be anything from 7-12 weeks depending on strain,usually around 8 weeks for most strains,so figure when you'd like your weed,take into account size,moving them around etc and figure when will be best for you