Reprint:
Traditionally marijuana has been harvested when the pistils die and the calyx starts to swell into a false seed pod. These days the best growers are getting much more detailed in their harvesting criteria. They take a close look at the trichomes themselves to judge peak harvest. Evidence that this is the only real way to tell peak maturity is in Sagarmatha's strain Matanuska Tundra, which ripens resin glands while most pistils are still alive and white. This seems an odd twist of evolution but proves that the pistil color and ripe glands do not necessarily have any correlation.
A small 25x or more pocket microscope, which can be picked up inexpensively at any electronic store, works well for taking a closer look at trichome development. What we are examining are the capitate stalked glandular trichomes, which will be a round gland head supported on a stalk. The coloration of these gland heads can vary between strains and maturity. Most strains start with clear or slightly amber heads which gradually become opaque when THC levels have peaked and are beginning to degrade. Regardless of the initial colour of the resin head, with careful observation you should be able to see a change in coloration as maturity levels off.
Some cultivators wait for about half of the heads to go opaque before harvest to ensure maximum THC levels in the bud. While you may be increasing the total THC level in the bud by allowing half of the glands to go opaque, the bud will also have a larger proportion of THC breakdown products such as CBNs, which is why some people prefer to harvest earlier while most of the heads are still clear.
Indica varieties usually have a 1-2 week harvest window to work with, while Sativas and Indica/Sativa hybrids may have a much longer period to play with.