For how long water will keep is often based on methods of storage versus the contents.
Water will keep forever; however, you will find that storage in plastic containers will limit your enjoyment. I find that if I rotate thru my 1L plastic bottles within a year they seem to still taste fresh. I think the real rule of thumb is no more than 2 years. I have a few cases of 4 year old water that frankly taste like plastic and only use to water plants.
For simple water storage needs cases of water work best, esp if you rotate your supply. For bulk water storage you might have to treat the water prior to storage to prevent such things as algae from turning your water supply unfit.
Remember when storing water just how much stress it will place on the flooring. One liter = 1 kilo of weight. Around big cities it is possible to find recycled 55 gal storage barrels that can be used to store water. However; not all used food grade containers are made equal. Water stored in these barrels should be treated first to keep you safe.
The fact that the water might have gone bad (taste like plastic, etc.); it can still be used for cleaning and other secondary uses.
For people who live in homes; the hot water tank from a water heater can be used as an emergency source of water.
Summary: Ease of use, cases of bottle water in plastic bottles work well. For long term high demand need, a secondary water system will be desired. Secondary static water storage systems often need to be treated for longer term use. Even after treatment, water should be periodically drained and refilled to keep it safe.
