You can flower them whenever you want. I have four that i veged untill they were like 20 inches and bushy (i cut alot of clones off of em), and i just started flowerin four of their clones like five days after i started flowering the four moms. So basically depending on the aims of your grow, and your setup then you can flower whenever you want. just remember this, you veg more to support more vegetative matter to support more flowers. You can do this by Either veging or by just adding more plants(S.O.G.).