If distilled water is in an airtight container, it will (hypothetically) be good indefinitely. The hypothetical part comes in with maintaining a proper seal, just not possible in a plastic container (at least as far as I know). The water will lose oxygen content, and go "flat" but the odd taste doesn't effect the drinkability at all. Water can be kept in plastic bottles indefinitely by treating it bi-annually with chlorine (plain, not scented), six drops per gallon, or two drops per liter/quart, as long as it doesn't evaporate, it will remain drinkable longer than you'll live.