Cannabis is harvested based on the length of the flowering cycle for each strain, starting from the date the plant is placed on a 12 on/12 off photoperiod (see Optimizing Marijuana Flowering). The length of the flowering cycle is determined by knowing the individual varieties you grow, and by observing the trichomes on the plant with a loupe or high-power magnifier. There are many differences of opinion on when to harvest a plant. Uncle Pete sees it like this:
Trichomes are clear until the plant reaches maturity. All cannabis experts agree on this.
As the plant matures the trichomes turn cloudy, and then amber. It is during this maturation phase that we harvest. The differences of opinion start here, and are usually based on the percentage of cloudy or amber trichomes present.
Harvesting our plants on the early side, or when the trichomes are starting to turn cloudy (25-50% cloudy), will result in a different flavor, smell, and effect with most strains. Harvesting them later (25-50% amber) will maximize these characteristics while ensuring yield.
All experts agree that harvesting cannabis before trichomes turn cloudy is too early. You lose potency, flavor, and yield. But that doesn't mean to continue flowering for as long as possible. It is much like a piece of fruit - we prefer it right before it starts to rot. That is when its the most flavorful and enjoyable.
Many excuses are made for harvesting early, but they are generally economic in nature and have nothing to do with the outcome of the harvest. Do not be fooled by these rationalizations. There is only one time to harvest cannabis - when it is done. We prefer to wait for harvest until 25% of trichomes have turned amber - 50% is even better with most varieties.
Again, this is a matter of opinion - your experience, thought, and consideration will guide you over time, but these recommendations will ensure that you have a good reference point.