You can top anytime .... I like to wait until it's good and booming before I put heavy stress on them but they are resilient little buggers and always bounce back we topped about 3 weeks in the first time but u can do it as many times as you need to keep you canopy level...
Usually they will wait until the 4th-6th node up before starting training this is because it can stress a small plant alot and could kill her if done too soon but anywhere after the 4th node you should be fine :) Best of luck my friend :)