I would guess many years. However, while we'd like to believe that bottled water was collected via boiling & condensation methods, there are horror stories (veracity of stories is left to the reader) of ordinary city water being bottled at the plants and distributed into stores for retail sale. Like other emergency supplies, water should be rotated through your stored inventory (replacing what you use) to keep it fresh.
Also, I read that the plastic used to make the cloudy translucent jugs are water vapor permeable. Thus you can have less-than-full jugs of water with the original seal intact.
The plastic used to make the clear bottles (e.g. water in plastic bottles like those used for Coke, Pepsi, & other soda pop) apparently does not leak water vapor and so does not have this problem.