Yea I waited till one was 15 inches with pistils sprouting all over, and the other one was 25 inches with no pistils coming out yet. Mainly dpends on how big you want your plants to grow, but remember flowering will likely double to triple the size of your plant, since mine have been flowering for 6 days now they are now, 15inches----> 29 inches ; 25inches---->39 inches. that is only after 6 days of flowering.. i think they are going to be around 5 ft to 6ft tall when they are done.