A standard hair developer expires after three years when stored unopened, in optimal conditions, and lasts for six months once opened.
Hair developer goes bad due to the breakdown of their active ingredient, hydrogen peroxide (H2O2), into water (H2O) and oxygen (O2), especially when exposed to light, heat, or certain impurities.
Therefore, manufacturers recommend storing the developer in a cool, dry place away from direct sunlight to maintain its effectiveness.
Photo Credit: hairfinder.com