Stole this from that other Idiots thread, since I reported it and may get taken down
If there is one person I think you can trust to give you the right info, it's Ed Rosenthal. I included an excerpt from a recent article.
Other very good professional growers that you may want to check out include Kevin Jodrey and Kyle Kushman. These guys started out as underground growers that have stepped into the age of science with this plant.
What Is The Best Way To Determine Your Cannabis Picking Time?
Watch the development of the trichomes.
Macro shot of immature trichomes.
Trichomes are the stalk-like resin glands that contain the active compounds, which grow on the leaves surrounding the flowers.
The flower area becomes covered with resin glands over time.
The length of this stage of growth usually lasts two to three weeks; in modern varieties these glands ripen in seven to nine weeks from flower initiation.
Late-season and long-maturing varieties usually spend about three to five weeks in this period of heavy trichome growth.
Macro shot of mature trichomes. Photo by Dynasty Genetics.
As flowers near ripeness, their caps swell with resin and the trichomes become more prominent and stand erect.
The viscous, sticky liquid that accumulates contains terpenes and cannabinoids, which are produced on the inside membrane of the trichome cap. As the resin accumulates in the cap, the flower odor becomes more intense.
The odor reaches its peak at the same time the trichomes begin to fluoresce in the light, twinkling like little crystals. In some varieties, the trichomes are so prominent that the whole bud sparkles. Using a magnifying glass, a photographer’s loupe or a microscope, monitor the buds’ progression to the peak of ripeness by watching the resin in the gland tops.
Stalked capitate trichome with bulbeous trichome. Photo by Professor P. Dyntasty Genetics.
Under magnification, you can see individual glands turning from clear to amber or a cloudy white. These colors indicate that THC is beginning to degrade into two other cannabinoids, cannabicyclol (CBL) and cannabinol (CBN), which are not nearly as psychotropic as THC.
When the trichomes begin to change from clear to amber or cloudy white, the buds should be harvested—this is the peak moment.
The Different Types Of Trichomes
Left: Stalked Capitate Glandular Trichome. Photo by Professor P. Dyntasty Genetics.
Stalked Capitate Glandular Trichomes:
These trichomes are the most abundant and contain the desired cannabinoids, terpenoids and flavonoids that growers seek.
These trichomes have no stalk and are much smaller than the other trichomes. They appear mostly on leaves rather than in the bud area, especially during vegetative growth, and contain cannabinoids.
These trichomes do not contain cannabinoids. They grow on the bottom of the leaves to deter pests.
Sessile Stalked Capitate Trichome:
These trichomes appear during the vegetative growth stage and produce only small amounts of cannabinoids.
Beautiful cannabis bud by Professor P. Dyntasty Genetics.
Ripe cannabis reeks of pungent terpenes and each day brings increased intensity of odor.
Rub the leaves surrounding the bud between clean fingers and inhale. This releases aroma molecules while leaving fingers sticky with resin.
Inhale and smell an exotic medley of familiar and unusual odors that may range from sweet to acrid with outlying musks and skunks.