Once hair bleach has been mixed, it should be used within a few hours. Mixed hair bleach has a very short shelf life because the ingredients are active ingredients. Once hair bleach has been used on hair, if there are any leftovers, they need to be disposed of properly and are no longer good. Hair that has been bleached is permanent, it does not fade or go away. However, hair that grows in will not be bleached and will be the natural hair color.