The soonest you want to top would be your second full set. Not single or 3 fingers, a full 5+ fingered set. As soon as you see the next set will be 5+, grab the next one.
One note on topping to remember. Top once, split the plant in two. This goes for that single main cola as well. What was once an XL cola will now be 2 lrg colas. Top one more time and get 4 med colas. Again and get 8 sm and so on. In short, each time you top, you divide that main cola so don't over do it or you'll end up with lots of sm --> med bud.
I personally top twice.