When you should start flowering basically depends on how long you want your cycle to be and what your max height in the room is. A plant will basically double in size the first few weeks of flowering, so if you flower now, they will be 24-26 inches when finished. Height obviously affects yield. Generally, although it is stain dependant, the bigger the plant, the bigger the yield. But I know people who go straight to flower after cloning, and people who veg to 36", so, its personal preference along with the limiting factors.