You can safely store water for six months to a year, depending on how it is packaged.
If it's commercially-bottled "spring" or "drinking" water, the American Red Cross recommends you can keep it stored for a year as long as the container isn't opened. Once the container is opened, use the water immediately.
It's much less expensive to store tap water in your own containers, it's recommended you store tap water for six months maximum.
It's a good idea to label and date the water bottles or containers.