Growing cannabis plants indoors require as close as possible a simulation of its natural habitat. Since the plants originally grow in sub-tropical areas, such as Afghan highlands or South American hills, they are used to long and sunny days. Using artificial lightning, you can emulate those exotic conditions virtually anywhere in the world, even in the northern latitudes.
Though a crucial factor, the mere amount of light is not the only thing a successful grower must look into. What matters is also the quality, or the kind of grow light that you use. Since the spring sunshine differs from one in autumn, you are strongly requested to adjust your lighting equipment and source to the plants' phase of growth.
Obviously the first thing you can regulate it with, is by using different light emitting devices (High pressure lamps such as HPS or MH, energy saving CFL lamps, or the cutting edge LED systems).
What happens to the light afterwards, however, is equally important - to distribute the light evenly and with as little loss as possible, you will need a professional reflector. Proper grow lights are a key factor to high yields - do not get tempted, to cut corners on it.