Bleach tends to lose strength with age, and rather quickly. What's recommended in some circles is to store calcium hypochlorite (pool shock) which can be mixed up into a bleach solution when needed. Store it dry, and when needed, it only takes a very tiny bit of the stuff to make bleach-strength liquid, which can then sterilize water without having had to worry about it losing strength.
Better than Bleach: Use Calcium Hypochlorite to Disinfect Water
A solar still is also an option for long-term water purification. One crude set of plans is:
Solar Water Still Plans