two months is optimal but some people grow 12/12 from seed. not all with good results but some do reasonably well. if you've got enough headroom for them th triple in size. veg for longer. if not induce earlier.
i'd say about a month minimum, but you can start 12/12 almost as early as you want, you just won't yield as much as if you had a bigger plant when you put her into 12/12
