It all boils down to growth rate. if your plants are growing crazy fast, then you need to flip them to 12/12 sooner then if they were growing slowly. So far for me, in my hydro scrogs, i can flip at about 75% full, and they'll end up being full and sexy. But my soil scrogs, i tend to let them fill in almost all the way before i flip. But, you need to really know your strain as well. Generally, the more sativa your girls, the sooner you gotta flip also, as they usually stretch more then indica dom strains. Hope that helps
-Live Love