Depends on the phenotype. If its auto it will flower itself if its seed it will need 3 weeks of veg before flowering regardless weather its a auto or photo. Clones can root quicker you can flower them after around 2 weeks but with a photo period strain you can keep it in veg forever if you want you have complete control on when the plant flowers by manipulating the light cycle. or changing it to 12 on 12 off.