Light
Light can be natural (outdoor growing) or artificial (indoor growing).
When artificial light is used, from the germination until the flowering stage, the plant typically remains under a regime of 16—20 hours of light and 4—8 hours of darkness, although the plant can use a full 24 hours of light without harm. It is worth noting that there is an ongoing debate over the importance of the "Dark Period". It has been proven that, when subjected to a regimen of constant light without a dark period, most types of flora, including Cannabis, will begin to show signs of decreased photosynthetic response, as well as lack of vigor, and an overall decrease in vascular development. There is no defined "flowering stage", unless you are discussing an auto-flowering variety. Typically, flowering is induced by changing the light schedule to 12/12, or 12 hours of light and 12 hours of darkness. Flowering in cannabis is triggered by a hormonal reaction within the plant that is set on by an increase in its dark cycle. In other words, a plant needs sufficient prolonged darkness for calyx development(flowering) to begin. Sativa generally takes as little as 8 hours of dark to begin flowering, whereas Indica can take up to 13.