12-18 inches is usually a good rule of thumb to induce flowering.
it depends alot on your strain.
i.e. sour diesel, if you veg sd to 18 inches then start flowering,
your gonna have a mini tree in your closet.
indica dominants you generally want to veg a little longer then sativas because
they stay short and stout, sativa dominants you want to veg a shorter time.