Aftercare

Leave the henna paste on as long as possible. Ideally 6 to 8 hours. The longer you keep the henna paste in contact with your skin, the darker and longer lasting your henna.
The heat helps intensify the stain, so keep your skin warm while the paste is on. To keep the skin warm yo can use Heating Pad, Warm Water Bag, or Space Heater. Try to avoid drying out the henna too much, keep your henna moisturized using Henna Lemon and Sugar solution.
Avoid perspiration and exfoliation of the area.
When you're ready to remove the paste, scrape it off with your fingernails or butter knife or a credit card. Do not use water to wash it off! Use a little olive oil or coconut oil or any other oil to remove any stubborn pieces.
Avoid contact with water for 12-24 hours. The longer you avoid water, the better the henna will oxidize
Initially the color will be orange (may even be nearly invisible on darker skin tones) and matures to a rich, dark red-brown or darkest brown or black over about 48 hours.

A well-cared design will last about 2 weeks. Ultimately, it depends on how quickly your skin exfoliates. These tips will help to prolong the stain.

Anything you do that is exfoliating will fade your henna. This includes swimming, cleaning chemicals, facial scrubs, and anything else that exfoliates the skin.
Avoid petroleum-based products, such as Vaseline, baby oil, sunscreens.
Protect the design with a natural oil such as olive or coconut oil each day, as well as before bathing.