Over time, hydrogen peroxide loses more and more of its effectiveness. It starts breaking down as soon as it is packaged before being used. Hydrogen peroxide decomposes into oxygen and water in a closed container. Water remains after oxygen escapes through the container's lid. Hydrogen peroxide can keep for up to three years in a closed container. It only works for one to six months after you open the jar. The reduced period is because the air hastens the decomposition of hydrogen peroxide.