According to HowStuffWorks.com, depilatory creams last about a week. They last longer than hair removal from shaving, but not as long as waxing. Depilatory creams work by being absorbed under the surface of the skin and slowing the hair growth in the areas where they are applied.
Depilatory creams work by dissolving the keratin in hair, which loosens it so that it falls out of the follicle when it is rubbed or washed away. The alkaline chemicals that dissolve the keratin are harsh, and some people's skin reacts badly to them. A patch test should be done on a small area of skin to check for a negative skin reaction before using the depilatory cream on a wider area.