A suntan begins to fade within seven to 14 days. The sun exposure stimulates the outer layer of skin cells to produce pigment, which is the cause of the tan. Skin cells normally shed every two weeks, and as they are shed, so is the pigment.
Individuals who want to keep a tan can either continuously expose their skin to sunlight, thus generating pigment in the new skin cells, or try to keep the pigmented cells in place longer. One trick to prolong a tan is to scrub off older skin cells before tanning so the pigment-carrying cells remain in place as long as possible.