For the time question: I have stored my water in a large glass container for weeks. Hmm, the fish in that glass container don't seem to mind the water being over 4 hrs old. Seems like a weird thing to say, not letting the water rat for longer than thAt.
As for prepping water, as I change salts each time I use trial and error. I do a 16 gal
change so I add 14 gals of water and the recommended dose for that. Put my spare heater in there and the pump I use for the water change and leave it over night. Next morning I check salinity and once it's ok, I add enough salt for the full 16 gals and let it mix a hour or 2. Then I add the rest of the water watching the salinity drop as I do it. I stop at around 1.0252 on my digital monitor ( I love that thing).
After the first time with the new salt I right down how many cups I added.
Mixing high initially then bringing it down is easier than tweaking salinity up.