The most effective self tanning products are the ones that list dihydroxyacetone (DHA), an isomer of glyceraldehydes, as an active ingredient.
According to the American Academy of Dermatology, they begin to work within around 40 minutes to an hour.
DHA is a dull (in coloring) sugar, which is important in the metabolism of carbohydrates.
It interacts with the dead cells located in the stratum corneum or outermost layer of the epidermis. In other words: DHA reacts with amino acids in the epidermis, producing a natural pigment called melanoidin that bonds with proteins in skin cells.
And results are a change of color that can last up to about seven days.
There are a couple of things to note here:
First: although melanoidin is produced, and melanoidin and melanin (the dark pigment that enables a sun tan to develop via UV exposure) work together in the absorption of harmful rays, a self-tan only provides about a SPF 2, more or less, not high enough to offer enough protection.
So take care to wear protective outer garments, eyewear, etc. and / or a sunscreen.
And second: note for a quick overview that in general, sunless tanning products that are out on the market today are reported to provide users with pretty much real looking tanning results.
From start of application of the tanning product until complete drying time (about three hours), you can have a sunless tan that can last all week. The chemical makeup of the products versus where and how they interact with the different parts of your body can affect tanning results; i.e. some areas may demonstrate different color variations and the tanning may last longer.
So: Where does the tan go?
Dead skin cells rub and wear off the epidermis, resulting in the fading or losing of the tan.
In fact roughly once a month or a period between 35 to 45 days, the epidermis is all rejuvenated. In order to keep a tan going long-term, many companies recommend that you reapply their sunless and self-tanning products approximately every three days. Some products last longer and wear better than others, too.
So experimenting, even with products that do not list DHA in the ingredients, might prove to be rewarding.