Ideally you should grow your plants from seed under 24hrs of light as MJ is a high energy plant that doesn't need a dark period to grow. The starting with 12/12 thing was a trick to induce the sex a little sooner, but I don't like to do that as it puts unnecessary stress on the plants. Allow them to veg under good light for 5-7 weeks and they will reach sexual maturity. You will know the maturity by the branches beginning to grow staggered from each other rather than directly across from each other.
After they reach that point, they will begin to produce little "pre-flowers" that will either grow out on a tiny stem with a bulb, and look like a tiny lollipop(male), or they will stay tight in the cusp of the spot where the branch connects, called the "node point", and will produce 2 tiny white hairs that stick out of the little bulb (female).
Once you see these, you can determine if you have any males that need to be removed (unless you want to produce a bunch of seeds rather than quality smoke). Then iff the plants are the size that you want, you can switch the lights to the 12hrs on/12hrs off cycle to allow them to begin the flowering phase.
I don't know how much you know about growing MJ but, You will need good strong lights that are in the correct spectrum of color. Not just any lights will do, as these are not house plants. Ideally you want to have lights that produce a color that is in the 6500Kelvin range for vegging plants, and you need enough to produce at least 3000 lumens per square foot of growing area. Many of us use T5 high output florescent lights ffor this.
Then when it comes time to flower, you will need to increase the lighting to 5000 lumens per square foot of growing space, and it is best to use a light that produces around the 3000Kelvin spectrum as that is best for flowering. There are lots of options for doing this depending on how big and how many plants you wish to grow.
As I said, I don't know how much you know but there is quite a bit to growing MJ as it is a very need specific plant. I suggest that you do some reading here under the different threads, especially the stickies as there is a lot of valuable info there.