How long can purified water sit in plastic jugs?

[SirCaptain]

I have a 5 gal plastic jug (see attached pic) that I have been filling at the store right before brewing. Well, SWMBO decided that she wanted a Berkey for our drinking water. To justify (in my mind) the ridiculous cost, I want to use it for brewing. The problem is that it would take a long time to get 7 or more gallons of water.

So, the question is: In a properly sanitized jug, how long can purified water sit? Additionally, given that I can only purify two gallons at a time, how is capping and then recapping the jug going to affect the time? Keep in mind that some of the water may be used as top off water (not boiled or even warmed beyond room temperature).

Thanks

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[Cyclman]

I would boil it before use, I don't think there's any way to know what might be on the spigot, in the container, etc.