Cannabis goes through 3 main stages of growth:
The first stage after germinating the seed can last up to 3 weeks. This is until the seedling is around 15-25 cm tall and starts to produce the first leaves.
The second stage is what is known as the vegetation. This is the stage when the plant will focus its energy on producing the leaves. This stage will last until the photoperiod (hours of daylight) is reduced to 12 hours. This change can either happen naturally outdoors, or by physically changing the number of hours in an indoor setup. It is important in this stage to focus on making your plant healthy and vigorous so as to avoid problems at the later stages. Once the photoperiod is switched, the plant will begin to start the flowering process. It will take 1 to 2 weeks before the beginning stages are clearly visible, and during this time the plant will stretch to about double the height (again varying slightly between strains).
In the third stage, the plant will focus solely of producing the flowers. You will be able to see when the plant is ready to harvest by inspecting the white coloured hairs. The optimum time to cut is when more than two thirds of these hairs turn to an amber colour.