you can flower whenever you want. many people have gone 12/12 from seed. remember though, that veg time greatly effects the yield so most people wait until around 12" or until they see pre-flowers. pre-flowers are a sign of maturity and are a good sign to go switch your lights over. if you don't have a lot of space or if your lights aren't very powerful, i would recommend flowering well before the 12" mark. more like 6" or 8"