once as Joshua mentioned, that the true leafs have started its up to you as to how long ya wanna veg em.  .longer in veg the taller/bigger they will get...i keep mine less time in veg to keep em way i like..unless im doin a scrog or mainlines.....some strains they say will "do this or taste like that" if ya keep em in veg longer, but its up to the grower really..   just dont chop em to soon at harvest time...GL