EXCELLENT question.

That depends on your grow area size. Some go for size/height. Some go by weeks.

The true answer is whatever works best for your environment.

If you let them get too big before the flower room, the bottoms will not get enough light and the plants could stress. Too small and there is a slightly lower amount of product. Some think it makes the plant stronger and more potent to be bigger, but this is not the case in my observations.

Myself, I let them get about a foot tall. Notice the word 'about', nothing is exact with me. I have a buddy that just got his card, and gave him some plants. Well he went a grew the heck out of them, doesn't have enough light, and is only penetrating about 2 feet. He, of course is thrilled, it is his first grow. Unfortunately, when he harvests and sees the difference between his grow and my grow of the exact same plant, I am afraid he will see the error of his ways. He needs at least 2 more lights in flower to keep up to these huge plants.

So, if I put them in at 12-16 inches, the sativa's get 4-4.5 feet tall, the indicas get about 3 feet high. These are under 1400 watts, most all is 9 weeks to harvest on average.

With all this jabbering, my point is, height versus light. Adjust timing your moving plants to flower with this in mind. Also, I have a perpetual garden,so this is a constant process.