node question

I have 4 plants growing in a space that is 2.2 ft * 2.2 ft* 4ft(l*w*h). They are in their 6th week of vegging under 175w MH, 100w HPS, 3* 26w cfl(6100k). They vary in size(6-10 inches) but all have similar amounts of nodes(5-. My question is, when is the earliest possible time I can induce flowering(12/12) with regards to the amount nodes the plants have. Im starting to get worried with the lack of space that I have.

you go do a seed or clone from start to finish with a 12/12 schedule if you want. so you can go to 12/12 anytime.
key thing to remember is the longer you veg. with at least an 18/6 schedule the bigger the finished plant but also more yield.
when you induce 12/12 the plant will get to be about 2 to 3 times its size when you go to 12/12, strain dependant of course.
if ceiling height is an issue just do a little tieing and training on them to keep them with a lower profile and they will get more budsites with a more even canopy.