Because the dye binds to the outer layer of the existing eyebrow hairs and can tint the skin underneath only lightly, the result is not permanent. How long it lasts depends on several factors: the depth of the shade you applied, how often your brow hairs naturally shed or grow out, your skin's cell turnover on the brow area, and how much oil, makeup remover or cleansing products touch the area.
In general, the visible effect decreases gradually rather than ending suddenly. The skin tint fades fastest, often within days, while the colour on the hairs themselves lasts longer and slowly washes out with each cleansing. People usually refresh the tint when the brows look less defined in the mornings and they find themselves reaching for a brow pencil or gel again.
There is no single correct interval — the right frequency is the one that keeps your brows at the level of definition you want. If you prefer a soft, natural look, you can apply a lighter layer for a shorter time and refresh more often. If you want a deeper, more graphic brow, a slightly longer application may be better for you, but the colour will still fade and need redoing.
To get a more even fade, cleanse the brow area normally but avoid rubbing the tinted brows hard, and do not use heavy oil-based cleansers directly on them more than necessary. Also, apply the tint only to clean, dry brows; any oil or cream on the hairs can create an uneven finish and make the colour shorter-lived.
If you are not sure how often is suitable for you, start with a light application and frequent refreshing. This lets you adjust before committing to a stronger shade.