How Long Hair Dye Lasts
A: The duration of the color depends on the type of hair color. Temporary color only lasts until the next shampoo. Semi-permanent and demi-permanent colors last 8-12 shampoos. Permanent hair color, however, is just that: permanent. It lasts indefinitely.
Some color choices (specifically some reds) may fade more readily than others, but in general, the colors won't need refreshing before the point at which you need to address the new growth issue. Because of this, you can simply do your retouch service at the usual time (5-8 weeks) and then bring the color through the hair at the end of the processing for the retouch in order to refresh the color.
The variance in the length of time between retouches is largely dependent on the rate at which your hair grows. The average rate of growth for the hair is one-half inch per month, but some people may have slightly faster or slower rates of growth. Feel free to simply gauge when it's time for a retouch service by when you see new growth in your style. This will ensure that your hair looks good on your schedule.
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See also:
Hair color and sun fading
The different hair color types
How to prevent red hair from fading too fast
Why does my hair color wash out so fast?