Bottled water storage

I purchase bottled water (5gal) for brewing due to my bad tasting tap water. I usually purchase a few and store them on the basement cement floor to keep them relatively cool.

Any suggestions as to how long one should keep these things before they go stale? Do they go stale?

I have a partially used bottle that I was going to use a couple of days ago when I thought of this. It's not expensive to refill with fresh water, just more of a matter of me being lazy.... er, I mean, more environmentally friendly.

I would think they would be good indefinitely (as long as they weren't opened).