Basically forever, however some contamination does occur and progress depending on how much entered the water from the time the water left the aquifer into the bottling process. Boiling that water would render the stored water drinkable again. Some concerns about leaching of plastic into water if bottles are stored for a long time – however, storage in a cool sunless area will minimize the deterioration.
The FDA considers bottled water to have an indefinite shelf life if it’s produced in accordance with regulations and remains unopened. Therefore, expiration dates on bottles are voluntary, and may reflect concerns for taste and odor rather than safety. Bottled water should be stored in a cool location away from direct sunlight.
Sources: US Food & Drug Administration