I have a 44 gallon Brute container that holds RODI. I have a bottom drain Tee'd off to gravity ATO sump with float switch, and other side to drain to my 20 gallon salt mixing container.
If I don't mix salt water or plan any water changes, then a full 44 gallon would last several weeks to a month, depending on evaporation and weather.
I do have a backroom with tons of space, so space is no issue for me.
Have a 120g tank with 50 gallon sump, probably only about 150 gallons of actual water.