No Title
The most important thing about knowing when to top, in my opinion of course, is after the roots have established themselves. I used to run hydro and it was easy to lift up the skirt and peek underneath to see if the roots have dropped into the reservoir. With soil/soilless it's tricky. I topped my girls at the 4th node in my first ever soil grow and they all responded well.
Here's what one of mine looked like a week or so after topping.