Opened bottles should be used as quickly as possible, no more than a few days (two weeks max). Opened bottles can attract germs, which can make you sick.
It's good to learn how to purify water that may not be fresh. You'll need coffee filters for that, as well as some other supplies. Also, a good tarp and bucket to help catch rain water, then run the water through several filters a few times to clean out any debris. There are extra steps to kill any water-related germs that might be hanging out (some recommend a few drops of bleach or boiling to reduce impurities), otherwise you'll be living over the trench pit sick as a dog.
Check out some of the survival blogs for easy ways to collect and purify water using household stuff most of us have. Hope it never gets that bad, but it's good to know just in case. Knowledge is power.
I’ve got an untested theory that a length of six-inch-diameter PVC pipe, several feet long, filled with the type of sand used in swimming-pool filters, capped and with a bung-hole at one end, would work well for removing fine particulates from water.