You are growing insde correct?
Under a light?

If you are growing 10 plants in 3 gallon pots under a 1000watt hps .. You don't want to veg for to long... actually it is not duration but rather height of your plants...
If you start from clone, you should flower when your plants are about 10-15inches tall..

Will you be cloning?

Now if you have a smaller light... 400watt or flurs.. then you want to flower when the plants are even shorter...

Indoor lights do not penetrate very deep beyond a plants canopy.. especially when you grow several plants next to each other... So letting a plant grow taller than 12 inches before you flower (indoor).. is not ideal...

unless you have SIDE lighting....

hope this helped