Water Can Keep Forever.
RO/DI water is water that is passed through semi-permeable membrane and filtered. This leaves pure water that is free of nutrients, contaminates, minerals and microbes. Since there is nothing in RO/DI water to cause a bitter taste or toxins over time, it can last virtually forever.
Metals are Released Over Time.
Although RO/DI water is pure, it will not keep longer than two years. This is because the container used to store RO/DI water releases metals or synthetic nutrients over time. Also, sometimes algae or fungi will pass through the filter. If the RO/DI water is exposed to light, this can cause algae or fungi to grow.
Bottom Line
Depending on how it is stored, RO/DI water can last for a very long time. Acid-washed and heated glass containers are usually the best to store the water because it is free from impurities that may contaminate RO/DI water over time. If the container is opened, this allows dust and other pollutants to fall into it and may cause a bitter taste. Although it has not been proven how long RO/DI water can keep, basic chemistry suggests that if it is left unopened and kept in a dark place then RO/DI water will last longer than one person will know.