Ive kept water for several weeks. Though if you do this, I would recommend checking the salinity levels before dumping it in your tank for a PWC
( should anyhow.. but especially after having it sit). If I use it within a week it doesnt vary much, but longer than that, water tends to evaporate and I have to add a bit of fresh water. Also make sure to have a power head or stir it up really well and wait a bit ( maybe 24 hours) if its been sitting for a long time.
this was treated tap water + salt mix, I just got an Ro
for christmas, so I'll let you know how that goes when I start making batches of make up water.