That very much depends on how large your grow space is, what the time constraints are like, in addition to how patient you are. The plants should double or triple their size during the 12/12 cycle. I believe general consensus is to wait at least until they're 12-18' before you put them into flower. Remember, the longer you leave them, the bigger the plant will be, the more budding sites there are and ultimately the more they'll yield. Happy growing!