I have used the salt water right after mixing, because sometimes it just works out that you are out and didn't mix some ahead but they need it now.
But in general, yeah, keep it mixed and ready to go.

Oh, when I have to mix and use right away, I warm the water to an acceptable hermie temp before mixing to help the salt dissolve.
I don't know about all salt having a universal mixing measurement, so I'd just make sure and read the instructions on the box. If it says 1 cup to 1 gallon, then you can use the Instant Ocean mixing directions on that web link.
