theres no rule but its easier to handle a 6 week old plant than a 2 week one. older ones dont mind being abused as much.
when theres 6 or so branches with several new meristems on it you'll be golden.
cut the stem at the stalk, scalpel preferred, clean blade, I use a small cutting board. final cut near new growth and scrape a bt to expose the cambium layer, dip in clonex and into a moist cup of pro mix/rooting cube etc. water in lightly mist , dome for a day, uncover and dont water till you see roots coming out. thats what I do for a 99% success.