How long does a bleach stay on skin?

It depends on the type of dye you are using. Most permanent dyes process in about 25-30 minutes. I would imagine most semi- and demi-permanent dyes are about the same. If you are using a vegetable dye (such as manic panic, special effects, or henna) you may leave on as long as you want, from 30 minutes to overnight. It does not contain chemicals that alter your natural hair color, but instead deposit color over the natural color that washes out over time.