plants generally grow about 3x their size after flipping the light cycle to 12/12. So a 6" plant would grow to about 1.5 ft if not more. You can even hold off switching the nutes any until you see pistils emerge. When you do flip to flower expect to see fast growth results then they will continue to grow slowly while getting fatter in the buds. The key really is the light change. Takes give or take a week or more and you'll see the pre-flowers and then the flowers will start to fill-in.

What kind of lights are you going to run for flowering?