According to NASA, a full moon technically only lasts for the moment when the Earth is directly between the sun and the moon. However, full moons appear to last for approximately three days.
Over the course of approximately 28 days, the moon goes through different phases based on the relative positions of the Earth, the moon and the sun. A true full moon only occurs during the instant when the moon appears completely illuminated to observers on Earth. However, when most people talk about the full moon, they are referring to the several-day period when the moon appears to be at least 95 percent full.