Most brow tints affect both hair and skin. The skin stain is often what creates the full, dense frame around the eye from the moment of application; as the top layer of skin renews, that tint fades over a few days. The colour deposited on the brow hairs lasts longer — commonly around one to three weeks, depending on the individual hair cycle, skin type, and after-care. The exact duration is personal, so you will usually see the edges and density fade before the hair colour disappears.
A few things shorten the visible result. Face washing, hot water, steam, facial exfoliants, and heavy skincare products around the brow area lift the skin tint faster. Oily skin also tends to lose the skin stain sooner than dry skin. The brow hairs themselves keep the pigment longer because the colour coats the hair surface, but newly grown brow hairs will be uncoloured and gradually make the brows look patchy again. When you see pale gaps at the roots or the overall brow shape has faded, it is time for another application.
To make the result last longer, use only gentle cleansing around the brows for the first couple of days and avoid rubbing the area with toner pads, acids, or other exfoliating products. If you want the colour to be lighter early on, you can rinse the dye off with lukewarm water; the tint on the skin will fade naturally anyway.