Several answers to this question. How soon can you start? You can start them 12/12 straight from a rooted clone, or from a sprout. Should you? Well, there are cases where that's a good idea (such as SoG). Another answer depends on your strain, and your size of plant. A plant put into 12/12 is going get 2-3 times bigger than it was when it made the switch. So, if you start a 2 foot sativa into 12/12 it may finish 6 feet tall. This is great if you have the vertical space, not so great if you don't. Most growers reccomend (and myself) vegging anywhere from 3-4 weeks from seed, or 1-2 weeks from rooted clone. In the end it depends on your space, and how long YOU want to veg it. There is no "right" answer to this question.
Thank you, i really appreciate your response. it helped alot. Well, my plants are 23 days old, and doing quite well, one plant is almost a foot tall, the rest at least 8+. So saying my plants were a foot and a half tall after one month, then i could still go ahead and flower? I am not concerned with yeild, just the fact that i can do it, and be proud of it. My vertical space is about 6 feet, minus about a foot of lighting, and then space before plants. at a foot and a half tall, that would most likely be the best time considering my restrictions, correct?