Bleach is one of those household chemicals that loses its activity over time. It doesn't matter whether or not the bleach container has been opened or not. Temperature is the primary factor affecting how long bleach remains active.
According to Clorox™
, the amount of hypochlorite that is added to their bleach depends on the season in which it is manufactured, because temperature affects the decomposition rate of sodium hypochlorite. So, more hypochlorite is added to bleach made in the summer than in cooler months. Clorox aims to maintain a 6% hypochlorite concentration for at least six months after the manufacturing date, assuming the bleach is stored around 70°F. It takes about 4-8 weeks from the time chlorine bleach is made to when it gets to a store so that you can buy it to take home. This leaves you 3-5 months where the bleach is at the effectiveness level stated on its label.
Does this mean bleach is useless after 3-5 months? No, because you probably don't need 6% hypochlorite for laundry and home disinfection. The 6% hypochlorite level is an EPA disinfection standard. If you store your bleach where it can get warmer than 70°F... like 90°F... the bleach is still effective for around three months.
So, when you buy a bottle of bleach, it has a shelf life. The bleach will be highly effective for around 6 months and fine for home use for around 9 months. If you're like me and have had the same bottle for a couple of years, it's probably time to go shopping.