Veg 3 weeks only?

You can Veg as long or short as you'd like starting from when the first node of leaves (not the cotyledon) have fully grown. It all depends on how big is your growth space/ how big you want the plant to grow considering the stretch during the first couple weeks of flowering ("pre-flower") which can range from 20-100% increase in size depending on the genotype (strain). *if grow space is limited choose the strain according to that parameter (the stretch) and which lighting you're using (to determine distance between the lamp and the plant). Good luck and happy growing :)