You can put it to 12/12 anytime you want, but you will get better results and a higher yield (Note: if you provide your plant with proper light and have enough grow space) if you Vegg the plant under 20/4 for 4-5 weeks. By this time your plant will probably show pre-flowers signaling that it is ready to produce bud at any time when you change the lights to 12/12. Your plant will average 3-4 feet if you do this method by the time of harvest (also depends on genetics and your grow methods obviously) and if you give it enough root space like a 5 or 7 gallon pot of premium soil with the proper nutrients, you will end up with a nice fat harvest.