I also think the experation date is more of a legal requirement than an indicator of shelf life. Assuming you start out with purified, bacteria free water, stored under normal condition (no sunlight, no extreme temperature cycles), I think the shelf life would be a lot longer than what is written on the bottle. I would almost say it's indefinite, or at least as long as shelf life of the container. It may taste funny or flat, but it will still be safe to drink. I've had bottles in my car for over 2 years, going through many heating/cooling cycles, with no problems.

Water that's been purified and refilled at home is a different story though. I don't think it's possible to get the water or container as sterile as from the factory, there's always a chance for a little bit of contamination. In an emergency, I'd probably still drink it, but I doubt it would last as long as factory sealed water. It might be still be safe to drink, but I know it will start to smell and taste funny a lot sooner.

With all the options for store bought water, why would you want to refill a container for long term storage?