How long does henna last?

Your henna stain will last 1 - 3 weeks depending on the location of the stain, your natural skin cell exfoliation rate and how well the aftercare instructions were followed.

The skin on your palms and bottom of your feet is the thickest (75 - 100 layers of dead skin cells). Thus the stain will last the longest here as it takes upwards of 3 weeks to exfoliate away the stained dead skin cells.
Moving from extremities to torso, the skin thins with less layers of dead skin cells to stain. Typically the back of your hand / foot will last 2 weeks; forearm / leg 1.5 - 2 weeks; arm / thigh & torso 1 - 1.5 weeks; neck about 3 days and face maybe 1 day.