That is entirely up to you and how long you want them to be in a veg cycle.
I recommend not topping. Take off the top two leaves of each branch, instead. It will free up light to get to lower branch nodes and basically do the same thing as topping without actually topping the plant. This means it will have a much shorter recovery time, if it has a recovery time at all.
Some people top each branch 5+ times, so don't worry about overdoing it. If you want a thick plant, all the power is yours! Grow her how you want.