Depends on how big of a plant do you want at the end. Most strains double in height when put into flower. So if you put a veg'ing plant into flower that is 15 inches tall you should end up with something around 30 inches at the finish. If you top the plants in veg that I have grow the stretch is about thirty percent. So a 30 inch topped plant going into flower should be about 40 inches or so at the finish. Most clones are ready at 10-12 inches, from seed 12-14 inches for a minimum height to get some decent bud but you will not have very big yield per plant.