~8 week vege.. she's gonna be big and beautiful, lol.
Figure they increase size by 2x or 3x after you flip, so your area and rate of growth will determine when you should switch to 12-12.
No worries, if it is going to be overcrowded, you can prune it back a bit.
limited info here, so take what i said and apply to your variables... when it fills up roughly 2/3rds LxW dimensions, that's ~ .5 (.7*.7=.49) and has room to double up after flip.
if too large, remove branches growing sideways or very low on plant, but i'd wait to see stretch first... you have time to be patient about it, so take your time on any pruning needed, if any at all is needed.