Depending on the strength of the dye, you're looking anywhere between two to three weeks for a henna tattoo. It also really depends on where the tattoo is, based on how often you wash that area and how quickly that part of your skin exfoliates.
Henna is a natural wonder that is used quite broadly. Aside from it's manufactured purposes in the cosmetic industry - hair dye and skin dye - it has also been used to help with skin irritation such as dandruff, eczema and skin surface wounds.
To get the best results for a longer period of time, you'll want to keep the area as dry as possible, disturbed as little as possible and moisturised. This will see that your henna design look good for longer.
It depends on what you're wanting to get henna for. If you're wanting a natural hair dye, this is a great option. Or, if you're after tattoo designs on your body that won't last forever, then henna tattooing is a great option. It is a safe and temporary treatment that produces a beautiful result.