The number of light-hours a cannabis plant receives in each 24-hour cycle is the chief determinant of whether that plant is in its growing or flowering phase.
Plants that receive 18 hours or more of light in each 24-hour cycle are in their growing or vegetative phase and in nearly all cases will remain in this state until the number of light-hours drops. They gain height quite rapidly. Vegetating plants will often display pre-sexing spurs at their internodes, but will very rarely show their gender. Their foliage is generally a lighter green than that of flowering plants. How long vegetation continues depends on the grower, the growing area, the variety being grown or the natural light cycle. There is no set length of time for the vegetative period.
Cannabis plants that receive 12 hours of light and 12 hours of UNINTERRUPTED darkness per 24-hour cycle for a period of two weeks or more will enter their flowering phase. Flowering Indica varieties will slow their upward growth in favour of producing female or male flowers. Sativa varieties will often continue their upward growth while producing flowers. Flowering plants (especially Indicas) often turn a darker green as they mature. The flowering period for each variety of cannabis is a set length of time, usually between 6 and 12 weeks.