if you start from seed the average wait for it to be safe to take clones (when they are too young, they sometimes just die on you), is approx 1.5 months... 8 weeks is better, but if it must be sooner, 6 weeks is best... also though the venerable jorge cervantes says that the clones should be taken from the bottom two limbs, you can and should take clones from all over your plant as some work better from the top and/ or middle sections.. some of the most successful clones that i've taken have been from the top section... each plant is different, so take the time and get to learn what your plants need and want when being trained for cloning... there are several great books out on mmj growing: cervantes, rosenthal, green and edwards have each written a great book with informative answers to all questions you may have regarding these questions...