you have to veg them til they are about 12 inches in hight. Thats roughly 3 and a half weeks. Any sooner and you will have poor results since they dont normally become sexually mature until then.
It is best to veg until after the plant starts showing alternating nodes, that is an indication of the hormonal changes that the plant is ready for reproduction. The longer you veg the more plant you will have to flower, it's worth hanging in there to 6-8 weeks of veg.