When is the earliest i can "top" my plants?

OK so the story goes i have a very limited area. I've done my first two grows just letting the plants do thier own thing. But now i'm looking to maximize yield. I've several opinions about when you can top but i'm wondering "whats the earliest that you top"?

well you can really top any time after the 3rd node.. this will make the plant very short.. thats what i did and i have 2 colas each at 5 grams dry, which is pretty good considering how small they are...