Welcome to growing man. It's a big part of my life now and I hope you expect the same.
Now, to get to the fucking pointttttttttttttt.
Stupid question? No. A NECESSARY question. Not everybody understands the 12:12 photoperiod.
Let me give you some background details about Flowering a Cannabis plant that'll make this more easily understandable! Cannabis is a plant that has 2 genders, male and female. Just like us humans. That seedless pot you're dreaming about growing is called "Sensimilla." Only 100% Female (with no hermaphroditism) will create Sensimilla. (Seedless bud.)
Males make seeds, females will make seeds if pollinated by males. Cannabis is also a fall-flowering plant, which means around August (depending on where you live) outdoor Cannabis plants will begin to flower due to the 12 hour days, and 12 hour nights of Autumn. I'm assuming you're growing indoors, so this is what you need to know:
1.) Before you put your plants in the dark, realize that the plant needs 12 hours of complete and total darkness with no light leaks.
2.) WOW 12 hours of dark, then 12 hours of light? Yes dude!! You need to fool that plant into thinking it's fall so it will begin it's journey to bud-production.
3.) Make sure you decide to put your plants in the dark at a reasonable time so you can check on them during the daytime. Personally, I throw my plants in the dark at 1:30am - 1:30pm.
4.) Continue the 12/12 light cycle for the rest of the plant's life. *OR UNTIL MALES ARE SPOTTED, WHICH SHOULD BE CULLED FROM THE GARDEN UNLESS BREEDING SEEDS!