Generally, the larger the plant the more bud you'll be able to harvest from it; however, to answer your question... You can put your plants into 12/12 any time after they have their first set of real leaves and in a few days you'll find out whether it's a boy or a girl. That said, I personally prefer to top my plants twice in a 30-40 day period while they're in veg (if I'm growing in a confined space indoors) and have a Sea of Green type thing going on so I can get the maximum yield from each plant once they produce bud(s).