It would be more ideal to veg for 1.5 - 2 months. The plant will start to show signs of its sex within this time and any time after that you can switch your lights to 12/12.
No offence dude but you really need to get some research done.
If you grow an MJ plant for long enough they will show their preflowers before you enter into 12 12.
You look at the nodes (branches) and if you see a cluster of little green balls its a male if you see little white hairs then its a female.
The reason alot of people have to wait until they switch to 12 12 is because they want to keep the plant small or smaller than it should be usually people veg for an average of 4 weeks and then switch to 12 12 it takes around 7-10 days for the plants to show their sex this way.
But like i said if you do a little research then you will find all you need to know.
I've barely seen male preflowers on the plants that I grow (aurora indica, Afghan kush), took more than 8 weeks just to see the smallest of one pod in the nodes. My females, they show after about 4-5 weeks, can't mistake the pistil hairs.
As said by everyone above.. This is the best way to grow.. If you turn the lights 12/12 before you see signs of sex it can stress the plant, it also give you a way bigger plant that will mature more quickly. Bigger plants = bigger buds and that's what we grow for.
There is a really good book worth investing in for info it's called Marijuana Horticulture Growers Bible, It's written by Jorge Cervantes. This guy knows his shit and it has helped me out no end. Try and buy a copy off ebay or something.