Demi permanent hair color lasts approximately one month, assuming you wash your hair once every day.

More Info: Demi permanent hair dye contains no ammonia, which is the chemical that allows for more drastic color changes with permanent hair dye. Ammonia "lifts" color from the hair; demi permanent dyes, therefore, cannot lighten your hair, since there is no ammonia. Demi permanent dyes are often used to enhance or deepen a person's natural hair color.

Despite the name, demi permanent color is a type of permanent hair color. Unlike semi permanent color, which washes out after approximately four or five shampoos and which does not result in a uniform hair color, demi permanents cover grey hairs more effectively and last longer. Dark shades in particular may last longer than the typical month.

If you want to get more mileage out of your demi permanent hair color, sulfate free shampoos can help. Sulfates are surfactants, or the reason why shampoo foams up, and can be very harsh on hair-essentially stripping the color out. Another way to extend the life of your demi permanent color is by not washing your hair every day. By going every other day, you may get a good two months out of your color.