If you're growing from seed...then once you get alternating nodes(not coming out side by side, but staggered instead)...your plant is mature and ready to flower. Topping, Lst or other training methods are a good idea to create an even canopy with multiple tops.
Remember the plant will have a fair amount of stretch...conditions and strain dependent...so look at the height you have in your flower area...and judge from there.
The type of light you use will also dictate the height of your plant. There is no sense growing a giant tree with a 400 watt light. The bottom 2/3rds of the plant will be almost useless except fluff for the hash pile.
Drink!