The double in size. LxW you figure .7x.7=~50% so, when they fill about .7x.7 of what you want, it's probably a good time to flip to avoid overcrowding.. trial and error a bit with each plant, though. Some may stretch more than others.
Figure at least 21 more days of vigorous growth once you flip to 12-12 before it tapers off.
Probably within 1 week or now? harder to tell in pictures.. use your estimate based on concept, not my perception of space in a picture.
it will surprise you. first week you may think i am crazy.. but after 21+ days you'll see :P