I start taking my clones about 2-3 weeks before I cut my lights back to 12/12 by then the mother plant is big & strong enough to handle the branches im taking off, which is usually around 6-10 of each strain, but I already know mine are females since ive been cloning them for a couple years. If i was you and didnt know what my plant was Id let it get at least 2' tall with some long side shoots come out, then I cut the light to 12/12 to see wether or not your plant is going to get the female hairs or the male balls. Once you see what sex it is turn your light back to 18/6 and take your clones. I start at the bottom and take my clones from there.