Topping can generally done when the plant reaches the 5th node. I try to do all of my topping a good 2 weeks before the flowering cycle begins. I generally top three times during the veg cycle. I have heard that you don't want to top in the flowering cycle as it does need some recovery time to complete the new growth or Y shape that you are hoping to accomplish. This usually stunts or slows down growth as it does the repair. I know people that have done it early during the flowering cycle and had it work for them but I don't want to stress the plant out when it is in the flowering phase.