Retouch Highlights
A: Well, since highlighting the hair is generally a bleaching process, as new growth emerges in the hair, you will find the highlights seem to move farther down the hair shaft.
Most people find that they need to have their highlights retouched every six to eight weeks. Since the hair grows an average of 1/2-inch per month, at the end of eight weeks you will have an average area of one inch at the scalp where the natural color is present before the highlighting color appears.
In some cases, depending on the hairstyle involved, you may be able to wait longer than normal for a retouch service. However, be sure not to wait too long, since you want to make sure your hair looks its best.
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See also:
Highlighting and highlights
How to do a retouch service on bleached foil highlights
Is it possible to retouch your roots and highlights at the same time?