So, you may be wondering... what about tap water? Homeland Security recommends households keep at least one gallon of water per person per day for three days in case of an emergency. You can use commercially bottled water, but you can simply store your own tap water. FEMA (Federal Emergency Management Agency) recommends storing tap water in clean and rinsed plastic, glass, enameled metal, or fiberglass containers. Once you have filled the container, it should be tightly sealed and stored in a dark, cool location. The water should be rotated out about every six months. It won't necessarily go 'bad', but you may get some algae on the container (gross) and there is a slight risk of bacterial growth after several months of storage.
The recommendation is to discard bottled water within 2 weeks after you open it, but FEMA's recommendation for how long you can keep tap water is really quite a lot longer. Personally, when I store tap water (e.g., for the hurricane season), I only keep it for a couple of weeks to a month. If the water starts to turn green, use it to water your plants or something, clean the container, and refill it with fresh tap water. Similarly, discard the tap water if it develops any other discoloration or has an 'off' odor.