You can flower whenever you want. Some people put it into 12/12 from seed. Most people veg. them for about a month + or - a week and then put them into flower
put them on 12/12 when ever u want , yes there gonna double or triple in size but i would grow it out for a few months on 18/6 to get a decent yeild because it takes a few months to get your harvest when u go 12/12 might aswell milk it.IMO there is no point of going 12/12 b4 a month because your plant wont start to flower regardless of light hours until it has matured
you got to take the time and learn yourself after awhile poeple will quit giveing you info if it doesnt look like your tryin to do it on your own half the fun of this is learning it yourself plus some guys on here will give b.s. answers and you wont have a clue and end up jacking your plants dont be lazy get as much info on your own as you can