If you are growing indoors, you should flip the plants into flowering when 2/3’s of your overall canopy is filled with vegetative growth. As the cannabis plants transition, growth rates will increase dramatically and the empty portions of your canopy will begin to fill quickly. If there is too much growth during the vegetative cycle, things will get out of hand during flowering.
If you’re growing outdoors, the change in seasons will automatically flip the plants into flowering for you. As the days get shorter (later summer/fall) plants will have access to less hours of light and will naturally transition into flower production. The earlier you put your plants outside (spring) the larger they will be when they flower and come to full maturity.