you can start the flowering period once the plant is strong enough to be put under 12/12 light.
people veg for different amounts of time, depends on strain, intended yield, amount of available space etc
As soon as the plant reaches it's fifth or sixth internode it is ready to flower, say after 3-4 weeeks growth.
it is even possible to put plants into flowering mode at seedling stage, although the yield will not be as large.
some plants will even start showing sex while in the veg cycle if they are mature enough.