I don't have enough experience to answer confidently so I won't even try but...

As it says in the bible

"As the pistils grow out from the calyx they form resin glands (trichomes), which gradually change in color before they shrink and wither. This withering of the trichomes affects the look of the pistils, which in turn fade and tend to look burnt and dry. This ^unhealthy' appearance of the trichomes is perfectly natural. The plant has completed the blooming period and the trichomes are no longer needed to gather pollen.The cannabinoid content of these trichomes is not lost, however. It is simply converted into other psychoactive cannabinoid compounds. THC will also degrade as it is converted into other psychoactive compounds. Eventually, exposure to light will further degrade these cannabinoid compounds to next to nothing if the plant is not harvested."

But it also says later down

It's all about careful observation of the trichomes and their comparison to withered ones. Don't forget that trichomes can be harvested early or allowed to grow past their peak in order to affect the eventual high of the finished product.