Howdy,
Approximately 4-5 weeks; maybe longer, depending on strain.
I figure you can take your pick of the following...
"Buds are at their peak potency about one week after flower formation slows...Harvest the plants when about half the stigmas in the buds have withered... When the plants are left in the ground, the resinous qualities of the plant may become more apparent. The bracts and tiny leaves may swell in size...The resin content of these buds may be higher, [but] the grass will smoke more harshly than if the buds were younger when picked." -- Frank and Rosenthal "Marijuana Growers Guide" pg 289, pg 295.
"In the primordial calyxes the pistils have turned brown; however, all but the oldest of the flowers are fertile and the floral clusters are white...Many cultivators prefer to pick some of their strains during this stage in order to produce marijuana with a clear cerebral, psychoactive effect." -- Robert Connell Clarke "Marijuana Botany", pg 140.
"Eventually the pistils start to turn color from pale white to red or brown...When the glands have swelled and the pistil has receded into the false pod, the bud is ready to pick." -- Ed Rosenthal, "The Closet Cultivator", pg 100.
"At the peak of florescence, all but the oldest of flowers have white pistil development...Another indicator is bouquet. When a plant is at the peak of florescence, it has a sweet and musky fragrance. Later, it loses the sweetness." -- Kayo, "The Sinsemilla Technique", pg 125.
"The best way to harvest is to examine the resin glands on each bud. As they turn from clear to amber, that is the optimum time to pick. Buds usually mature from the top down, if grown under artificial light, and you will end up with more high-quality p ot if you pick each bud when ready. However, the plant will not just continue to produces buds at the same rate. Like any other plant, the flowering cycle lasts a specific period of time. If you wanted a further harvest of buds, the plant would need a second cycle of vegetative growth. This can be achieved indoors by simply turning the lights back up to a 24 hour cycle for a few weeks. Outdoors though, you are dependent on the seasons. Frost and long nights will usually kill the plant. Of course, such a strategy is only variable if growing a few plants. If your operation runs on an industrial scale, just drive the combine harvester through the field." -- (-peter alt.hemp posting)