Hair bleach lasts for 20 minutes after it has been mixed. It must be applied immediately after mixing because it starts to lose its strength soon after mixing.
Hair bleach contains hydrogen peroxide and bleaching powder. When bleach has been mixed, it starts to oxidize immediately. This makes it lose its strength and effectiveness. Mixed bleach that is leftover cannot be used and must be disposed off.
Some brands of bleach may last longer after the bleach has been mixed as the ingredients differ from one manufacturer to the other. The instructions for use are specified on the package and include information regarding the time the bleach will last after mixing. Bleach that has not been mixed has a longer shelf life; the expiry date is mentioned on the bleaching kit.