Most peeps veg until the plant shows sex, this can be around 5-6 weeks. How tall they are then will depend a little on the kind of light they have had in veg. Bluer light will produce slightly shorter internodes. But I would expect them to be around 2 1/2 ft to 3 1/2 ft at that point. Not sure how much these will stretch once they are on 12/12 but 7 ft headroom sounds plenty to me.
Hope that helps a little.
EO