According to Hair Finder, you should touch up your highlights every six to eight weeks. While the color of the highlights is permanent, the color will move down the hair shaft as it grows.
Hair Finder asserts that hair grows an average of 1/2 an inch every month. So, at the end of an eight-week period, the hair has about an inch of new growth. This makes it noticeable that the highlights have moved further down on the hair strands. Depending on the type of hairstyle you have, you may be able to wait longer than eight weeks to have your highlights touched up.