My plants are about three weeks old and are either three or four nodes high about six inches. How big do they have to be before i can turn th elights on 12/12 to determine their sex.
essentially, whenever you want. you could even go 12/12 from seed if you wanted to. now, if you just wanted to find out sex and not necessarily flower right now, i would wait until either they just started showing pre-flowers, or take clones when you're able to and put those clones on 12/12 and see what the sex comes out to be.
It can take from a few days to over a week for a clone to root. After which the plant will take awhile to start growing again. About 2 weeks. You're better off taping a bag with black duct tape then covering it for 12 hours a day. That branch will start to flower and you'll know the sex of the plant before placing in 12/12. The longer you veg the more yield and potency.
You can change now, but forcing flowering w/ 12/12 can take up to 2 wks to have an effect on an immature plant. If your plant is three weeks into veg, and under 24/7 then it'll probably start to preflower in the next week or so. Your best bet is to be patient at this point. You're far enough into veg that you're not gonna save much, if any time, by switching to 12/12 early, so you're definitely better off not stressing the plant. Remember, you're hoping for females, and stress=less females.