Looking good! Nice plants! To answer your question...Switching them to flower kind of depends on how tall you can grow them. Assuming you're indoors as the pics suggest and height is limited then I'd start flowering them at the first sign of multiple branches. In general they'll grow anywhere from 12 to 24 inches when started to flower this early ( I do this and get about 1.5 oz's per avg. 18inch plant (each plant has 3 to 4 good colas). Rooting will progress rapidly as you fertilize with flowering nutes (the phosphorus and potassium are root food)
With no indoor height restrictions I'd veg them for a month or so until well rooted and branched feeding them growth food, then at about 18 to 24 inches switch them to 12 & 12hours of light and watch as they elongate a bit more and stack on the buds finishing at around 30 to 36 inches about 8 weeks later.
More vegitative growth/height than this seems a waste of time and plant energy. Taller plants don't necesarily make a lot more bud, well branched ones do. Taller takes time & = a slower turn around. I like to rotate 3 at a time, meaning cut 3 clones, move 3 rooted clones to flower, harvest 3 mature plants. Because I flower them after 2 weeks of vegitative growth i.e. 5 inches tall, I can harvest 3 plants every 2 weeks on a regular basis...I can't smoke it fast enough at that rate...though I try
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Out doors naturally they just do their thing. I still try to form as many bud sites as possible by bending and pinching tho.
Hope this helps, Happy Gardening! GrowIT!