If by LST the meaning was bending down the new growth branches, imo it can and should be started on Day 1 or Day 2 after topping. The only limiting factor on LST is your own patience and skill. If you overdo it too fast, you will break branches. If you wait too long, it becomes hard to bend the branches, again creating a problem of broken branches or disorganized growth.
Like most things experience is king. Dont start LST if you dont plan to mess it up a dozen times or so in order to learn. When a broken branch you fix recovers, its a great experience imo. If one is unfixable, you discover new ideas. LST to me is the best part about growing...hands on.