Heat and light are the enemies of chlorine. Store it in a cool, dry, dark place. Anything else, and the concentration of chlorine will drop over time. So, if you are storing in bulk, store it inside in a closet.

Of course, it's probably OK to store bleach for a few months outside, as long as it's not in direct sunlight. 6% bleach being degraded to 5% bleach will rarely make a difference. However, 12.5% bleach degrading to 6% bleach can ruin your day.