The best way to tell if the plant is ready is to examine the bud. In the paragraphs below, the terms "pistal" and "stigma" refer to the white hairs in the center of the female bud. The term "calyx" refers to the pod that would surround the seed (were the plant to be fertilized). Many growers elect to pick each bud individually, as it reaches it prime.
"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 around 80% of the pistil hair has turned brown THC is at its peak percentage... When the glands have swelled and the pistil has receded into the false pod, the plant is using the THC in the bud to produce food"
Ed Rosenthal, The Closet Cultivator, pg 100.
