Once your plant has completely brown pistils, health is declining and THC is breaking down into CBD and CBN, two non psychoactive cannabinoids.
At this stage, peak THC quality and quantity has passed, which is ok if you prefer sleepy/lazy type of stone, but if you prefer a trippy/mind expanding high, harvest ASAP or in the future, when there are still 5% whitish pistils left. Pistil colour reflects exactly on plant maturity and health, and it is plant health, not trichome colour, that determines cannabinoid chemistry.
Hope this helps,........
Organoman.