dude, depending on what plans you have with your plant. You can top it now, yesterday or tomorrow, if you know what I mean. Usually wait for the plant to grow about 5-6 levels, then consider topping. Classically, you top above the 3rd or 4th node. From there, depends on what you want to do. There are so many and so different techniques, but it's all about what you want and what interests you have - if it's a living sculpture, or if you are after symmetry or horizontal growth or different methods of mainlining. It all depends on how long you plan to vegi as well.