Expect your henna design to last for 1-3 weeks. The duration is highly dependent upon how well you care for your skin. If you keep the stain moisturized and keep it from rubbing off on things, it may last three weeks or even longer. If you don't care for the henna at all, it may begin to fade or peel within the first week.
The longevity of the henna stain also depends where the design is located on your body. The ink tends to show up darker on your hands and feet, but those areas also tend to exchange the most friction as you interact with your environment.
Moisturize. Apply a coat of a natural oil, butter, or lotion after the paste is removed. While the henna is on your skin, moisturize regularly* to protect the design and prevent exfoliation. Many store-bought moisturizers contain chemicals that can prematurely lighten the stain, so it's best to use something natural.
*optionalDo not use moisturizers that contain bleaching agents and/or fruit acids (Alpha-Hydroxy Acid). These chemicals tend to strip your skin of moisture and nutrients, and they can make the henna fade prematurely.
Spread a coat of essential oils over the design. Oils will keep your skin moist, which may prevent the henna from fading or flaking prematurely. Try using wax lip balm, coconut oil, or olive oil. Look for specialized henna-care oils.
Try not to rub off the design. Exfoliation can fade the henna. Rough washing and friction from clothes can also lead the stain to disappear more quickly. The less you touch the area, the better. If you have a henna design on your hand, consider wearing gloves when you wash dishes.
Clean your skin with a gentle soap. Apply with your hand or a soft towel. If possible, rub soap around the edge of the henna design, but not into the stain itself. Avoid using acetone (found in nail-polish remover) and hand sanitizers. These relatively powerful chemicals strip your skin and make the henna stains fade more quickly.