Depends a lot really on height restrictions, and the fade of CFL bulbs. Also look out for alternating internodes to tell you they are ready. I'm assuming this is your first grow you will not want them to get that large with CFLs due to light penetration factors lower on the plant. What is the true wattage you are using? What strain are you growing that will make a big difference in what height you will need to flower them. I have flowered plants from seed anywhere from 12"-30". If your using a slower growing indica let it veg longer you'll be a lot happier yeild wise.