I flip anywhere from 18 -24 inches usually. The main thing is to know when it is sexually mature. You will get alternateing nodes, which is to say that the branches coming off of the main branch will no longer be directly across from each other one side will be a little higher or lower than the other. Another way to tell is to watch for pre-flowers, somewhere from the 4th node up. Where the branch meets the main stem you will begin to see a little nub there. If this nub produces pistils, or 2 little hairs, it is a girl, if the nub developes more little nubs, or doesn't produce the hairs, then it is a male.
Anytime after the plant has become sexually mature, you can begin the flowering process...it can be from as little as 7 weeks to as long as 14 weeks and even longer depending on the strain.
You can not tell if the plant is finished by the pistils. Many other things besides ripeness can casue your pistils to change color. Watching the pistils is not a good way to tell when your plant is done. The only sure way to tell when the plant is done is by watching the trichs.