My Afghani-Durbans almost seemed to be ready by September 1 this year. New flower clusters showed up so I waited a couple more weeks. But, then more flowers started to appear again. Eventually, I clipped them on September 19. My friend had even more late flowers on his Thai marijuana plants. Should have given them more time?
As soon as the flowers mature, THC starts to decrease in concentration because of the light and heat around the plant. Even so, you should always keep marijuana plants that have new growth in the ground unless there is a risk from the environment, the law, or thieves. You could even just leave a few small buds to continue vegetative growth in the plant. It may produce an additional growth period.