There are different schools of thought on this topic. Depending on your method of growing(i.e. flood and drain, soil, aeroponics, room height, light amount,etc.)
Some growers that grow in high volume will flip their light schedule as soon as their clones show roots. I do not agree with this methodology. I am a firm believer in the reproduction of nature. In nature, a cannabis seed germinates
in early spring and grows in vegitation for several weeks before the beggining of Autumn. So in my opinion(take it or leave it) AGE of the plant is more important than height.Kind of like a grape vine for producing wine,the older the vine,the largrer the yeild. I don't flower until the plant is at least 7-8 weeks old.
This practice allows the plant to develope a strong root base and stalk.this allows for more nutrient uptake and larger yielding plants.
I "supercrop" my plants. This means I bend the top stalk lower than the bottom shoots by pinching the stalk between my fingers, approx. 3-5 nodes from the top. This allows the bottom shoots to catch up with the top shoot. When you do this, it must be done in vegitation and up to the third week of flowering. After that the stalk becomes too strong and snapping can occur. This is a means of "tricking"
the plant into thinking the bottom shoots are now the top shoots. A hormone is delivered to the secondary shoots as if they were the tops,creating a plant with multiple "king kolas". In this method I keep the plant at about 20" - 24"
before flowering. Otherwise the plants would be nearly double that height.
Rememeber...your plants will grow(depending on the strain,sativas tend to grow higher do to their longer flowering times than indicas) 2-5 times the height it was in vegitation by the time it's done flowering.
So basically, figure out how much room you have to grow, veg the plants to a height that will allow for double at least, and flower those babies.