if you do any kind of searching for "topping" youll find a plethora of info. If you are growing from clones (which is mandatory for SOG) then you really only need to grow them long enough to establish roots (which if they are good sized healthy clones with roots already, means only 1 week of veg). A 6" high clone in one week of veg can get nearly 10" tall. In 8-9 weeks of flowering, they can get 3 feet easy. Doubling and Tripling of size is normal.

For seeds, its as soon as the plant is mature enough to sex. This was my first time using seeds. Clones take a LOT less time and you can leave your garden during the beginning for flowering since you wont have males to look for. With seeds, I had to flower my plants for 2 whole weeks before they finally all sexed (5 females and 8 males). It wasnt until they had reached 7-8 nodes that any of them began to show sex. That was about 4 weeks of veg from when I planted them.

These plants are all sexed, so that gives you an idea of how small they can be.