How long can you keep RO?

until it gets contaminated with something.

RO/DI is pretty much pure water its not going to grow much on its own or go bad unless something gets introduced into it.
 
Theoretically, as long as it is covered, and doesn't get foreign matter introduced, it should keep indefinately (well, except for evaporation)
 
Keep it covered and in cool, dark place.

Also, don't keep 55 gallons if you only need 5 gallons at a time.

The more turnover you have the better it will be. If you use one gallon a day you'll be replacing 20% every day so it will stay "fresher".
 
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