A new bleach solution is prepared on a daily basis. This is also subject to the type of container in which the bleach is stored, but the problem is with the containers from which you have to pour from a bottle. Whenever the bottle is opened, air from the atmosphere comes into the bottle and causes the bleach to decompose, especially bleaches using hydrogen peroxide. This is also the reason that bleaches do not have an actual shelf-life of more than a month. After a month, they're just sterile water.