In the absence of continued exposure to sunlight, a tan begins to fade within a month and continues to fade for several weeks. A tan fades because the skin cells that were exposed to sunlight and developed pigment mature and fall off. Skin cells have an approximate 30-day lifespan.
Skin cells are continuously shed from the surface of the skin and replaced from layers underneath. When skin cells are exposed to sunlight, they produce a pigment, called melanin, which acts to protect the cell from the ultraviolet radiation in sunlight. Once formed, the melanin remains inside the cell. However, as the cell matures it eventually sheds off. Once all of the melanin-bearing cells are shed, the tan is completely gone.