In my oopinon it depends on the plant itself. Mine are about 2 months and in the preflowering stages but i was still able to detirmine sex, tho the dude about me has got it right, the best way it to wait till the 12/12 light cycle, if is a male youll see little "footballs" at each internode, if its a female youll see little teardrops with 2 white cloudy hairs (pistils) comin out of the teardrop.
i have the same issue i just germinated my seeds,. how long do i wait or what do i do to find out the sex,.. and how long do i have until the males ruin the rest of the plants,.. my plants are all together in 1 clear plastic box,. and also can some one tell me how should i keep my plants they just sprouted out the ground should it be total dark for few days?
If you look close at the cannabis female it has two sharp like fingers pointing up the stem.The major part of the tim if a plant has this it is female unless it is hermi.But if you have a true hermi that has happened just from seed without you causeing it it is worth has much has a pure female because this is the way to get femanised seeds.Put it in with one female and the seeds will be 100% femanised but it has to be a hermi from seed born that way.
During veg, you can do 12 hrs of regular light and 12 hrs of blue LED to reveal sex. I like to use computer fans with the blue LED. After that you can resume 24hr light veg with no problems whatsoever.