How long should you keep amla oil in your hair? As with most topical oils, you can use amla oil overnight, as a more intense treatment, or just use it as a mask, for 15-minute intervals. If you do use it overnight, try applying just once every couple of weeks.How long should you keep amla oil in your hair? As with most topical oils, you can use amla oil overnight, as a more intense treatment, or just use it as a mask, for 15-minute intervals. If you do use it overnight, try applying just once every couple of weeks.