In the abstract, if you have a clean container with nothing but water in it, and the container is sealed, the water should keep indefinitely. It starts with no bacteria. It is sealed, so no bacteria can enter. And there would be nothing in the container that any bacteria could use as food, to multiply. But that scenario is unrealistic.
Suppose that you buy bottled water, and keep it sealed. Will the water keep indefinitely? Perhaps not. A study of 57 different sources of bottled water, plus four samples of tap water, showed that many brands and types of bottled water contain bacteria, even in high amounts. Each water sample was tested for CFUs, colony forming units of bacteria. Each bacterial colony on a growing medium indicates at least one bacterium (because one or more bacteria are needed to give rise to each colony).
Fifty-seven samples of 5 categories of bottled waters were purchased from local stores. Samples of tap water were collected in sterile containers from the 4 local water processing plants…. The bacterial counts in the bottled water samples ranged from less than 0.01 CFU/mL to 4900 CFUs/mL, including 6 samples with levels substantially above 1000 CFUs/mL. In contrast, bacterial counts in samples of tap water ranged from 0.2 to 2.7 CFUs/mL. (Fluoride and Bacterial Content of Bottled Water vs Tap Water)
Different terms are used to describe commercial bottled water: spring, artesian, purified, drinking, and distilled. Only the term “distilled” has an exact and reliable meaning: the water was evaporated and condensed as a method of purification. The other terms are somewhat vague, and mean different things when used to describe different company products. Here is a table of the data from that study of water samples:
Every type of water had some examples with low bacterial counts. But every type, other than distilled water, had some examples with high bacterial counts. (See also: Study Finds Some Bottled Water Has More Bacteria And Less Fluoride Than Tap Water)
The take-away from this study is that commercial bottled water may have significant amounts of bacteria. But the study also indicated that distilled bottled water is probably your best bet. None of the distilled water samples contained much bacteria. The study also indicated that tap water is often safer than bottled water.
Now suppose that you are storing water long-term, and the water contains some bacteria from the start. What are the implications? First, the bacteria cannot multiply without some type of food. So if the container is very clean, then the bacteria would have nothing to consume and they will die. The bacterial count should decrease, not increase. However, even a slight amount of material left in the container could grow bacteria. And it is difficult to be certain that the container is clean enough.
So if you are storing tap water long-term, you can hedge your bets by treating the water before storage AND just before use. Water disinfection with household bleach is probably the least expensive and most available method, especially for water that is known to be relatively clean and safe, except for the possibility of some bacteria. For the details, see our previous post: Water Purification with Household Bleach.
For small amounts of water, use 1/8th teaspoon of bleach per gallon. For large amounts of water, use one teaspoon per 8 gallons or one tablespoon per 24 gallons. These numbers are approximate. So you would be fine using one teaspoon for 5 to 10 gallons, or one tablespoon for 20 to 30 gallons. The important point to remember is that, for long-term storage of tap water, treat before storage AND after long term storage. For commercial distilled water that is kept sealed, you only need to treat after long-term storage, prior to use.
See this EPA Emergency Disinfection of Drinking Water tract for more information.