Knowing that the right time to harvest has arrived is one of the most challenging aspects of growing your own cannabis. It's an exciting time, but if you harvest too soon you'll miss out on the highest production of resin, and if you harvest too late those same resin glands start to degrade. With so much depending on harvesting your cannabis at the right time it's good to have some knowledge about the biological changes each plant goes through.
You should become familiar with the pistils, which are the long hairs that grow from inside the calyxes of flowers since it is a change in their color that lets you know that harvest time approaches. At first, they are thin, stringy, and white, but they darken until they turn rusty colors of orange, red, and brown, and this signals that the flowers have reached maturity.
However, the pistils of mature flowers aren't the only thing you should monitor. To produce cannabis with the highest levels of a strain's particular blend of chemical compounds (cannabinoids, terpenes, etc.) you'll need a handheld microscope (30-100X) to monitor the development of trichomes, the sticky glands of resin that cover the flowers and produce those compounds. At first, the trichomes will be clear, but check them daily and you'll see them swell and elongate (some even form a little “cap” on the end) before turning cloudy, opaque, or white. This change in color tells you that the trichomes aren't producing any more chemical compounds, and the flowers are ready for harvest. Harvesting your flowers during this time is critical (preferably in the morning) because once those fragile trichomes change color again (to an amber color) it means those precious chemical compounds have started to degrade and lose their potency.
We must also keep in mind that pistils and trichomes can sometimes develop at different rates depending on where they are on the plant, so it's best if you check several flowers each day. No matter what, don't get discouraged; growing your own cannabis is a continual learning process, and with practice, you'll get better at spotting the signs for harvest.