Anything is possible, there is talk of people pulling down 2-4 lb plants out of under current systems, i have not seen the evidence but i would imagine they are trees. If you grow like most of us with just a short veg (a month or less) 1 to 4 oz seems common, veg longer more is possible, shorter for less per plant, more plants per foot.
I think he meant that after 21 days the plant has reached sexual maturity enough to support cloning, i prefer a month but that is up to you. And of course you can clone from fem seeds, just watch for hermies.