I used to make 100G of saltwater at once and then use about 30G or so once a month. So some of the water would sit for a few months before being used and never had problems. I did have a small PH going the whole time though.

I have tried it without a PH once and the water kind of turned a funny color after a week. Maybe just a freak thing that time though.

Saltwater, once mixed, should not settle out any salt. I've stored mixed sw for months before with no settling.