Wait until your plant has 2 nodes Meaning 2 sets of leaves if you want 2 main colas, wait till 3 nodes for 4 main colas, I waited till 6 so as to allow branches to grow from the sides first and get 4 main colas as well
You could top your plant when it is only a couple inches tall; it just would take a hell of a long time to recover. When topping, you want a strong root sytem to support the branching. I followed UB's advice on topping on my current grow (https://www.rollitup.org/advanced-marijuana-cultivation/151706-uncle-bens-topping-technique-get.html), waiting until there were 5 nodes and then removing all but the lower two sets of leaves. My plants recovered in a little over one week. These plants were 10-12" (cut down to 4"), so the short answer is YES, you can top at that height and they will only need a short recovery time.