It takes the moon 29.5 days on average to cycle from a new moon to a new moon. Sometimes, a full phase can be delayed or extended slightly when the sun's gravity effects the moon's orbit.
The moon has four basic phases: new moon, first quarter, full moon and third quarter. Since the moon doesn't emit its own light, it's only illuminated by sunlight. The dark section of the moon is the portion that is facing away from the sun, and only during a lunar eclipse is the Earth's shadow cast upon the moon. From the sun's perspective, the moon is always full.