Great question — I wondered the same when my highlights started looking brassy and dull after a couple of months. Personally, I try to refresh mine every 8-10 weeks depending on how they fade and how my roots grow out. I once waited too long and regretted it because the color turned uneven. When I needed advice on highlight maintenance, I found a fantastic hair color salon through hairstyles for women . Their pros gave me tips on toning shampoos and gloss treatments that extend the life of highlights. Worth checking out if you want to keep the color fresh without over-processing!