Don't go by height. Wait until the plants are sexually mature. which usually occurs after 4--6 weeks of veg.
Wait until you see preflowers. You're going to want to look for these where the branches come off the main stem. A femail preflower looks like a little cone/pod with two little hairs coming out of it. Sort of hard to describe but once you notice it you will see what I mean. And don't mistake new leaves for preflowers.
