All depends on how your growing.soil aero hydro,scrog,sog, seeds,clones.I like to veg 10 days for aero which gets me to about 14 inches then finish around 3 feet tall.Soil I veg for about 3-4 weeks to about 14 inches then go 12.It all depends on your wants needs and abilities.Use your height but dont underestimate the flower growth