Ideally, permanent hair color would stay just as vibrant as it was the day you colored it, at least until the roots grow. On most heads of hair, that would mean a good 6-8 weeks between touch-ups. UV rays from the sun break down the chemical bonds that are present in dyes, which causes the dyes to fade. That's why if you have color-treated hair, and you spend a lot of time in the sun, your color will fade more quickly the more it's exposed to sunlight.