Depending on how long the paste was left on, where the tattoo is located, and the natural properties of the individual's skin, henna stains will last anywhere between a week and a month. 

Thinner skin like tops of hands (which are also washed frequently, making them exfoliate faster) usually last around a week.  Arms and legs usually hold the stain around 2 weeks, give or take.  Stomach areas (like pregnant bellies :) can hold the stain for 2 weeks or so. Tops of the feet can hold the stain 2 to 3 weeks.  Thick skin such as palms or soles of the feet can hold the stain for a month or more, depending on the person.  

Upper torso skin (chest/upper back and above) is thin and much more oily than the rest of the body, and generally does not take the henna stain as well as other areas. 

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How does Henna work?

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/ PPD?