He said 15-20 cm's. Which is about 6"-8". My plants are close to twice that height, in little over 30 days. But I am sure the strain of plant and such is a factor also. I just put mine into flower stage almost 3 days ago. Seeing little white hairs come out of the nodes on my two talliest plants already.

I had my lights set on 24/7 when they were vegging, now they are at 12/12 with Power Flower nutes.