Depends on what you want and how many times you plan to top it after the first one, how long you want it to be in vegg.4-5 node not counting the bottom. How patient are you. I had 4 nodes on the plant in my sig but I've seen em topped much earlier
When I top plants I cut with a new single edge razor, I make my cut about a quarter to a third of the way up from the internode I plan to keep alive on the plant.
Typically you wait till the fourth internode has developed and the plant is well rooted before doing much cutting, topping, or trimming, etc. For me well rooted is roots filling out a 4x4x3 inch pot. This ensures the plant is strong enough to survive a bit of abuse without much issue at all. You can top earlier but the less developed the root structure the more stress the plant will experience.