Uncle Ben's topping guide says to wait until the plant is 5-6 nodes high, then top to right above the second real node for four main colas. I've used this technique myself, and when I first did it I was terrified I had murdered my plants. But I did it right, and they flourishes with four main colas.
I only topped once, though. Other people top multiple times. You gotta figure out what works best with the strain you're growing.