Keep it with a lid on it and mix it before using . Mine is mixed and usually last a month but I cant see any reason why it would it wouldn’t last longer . Hope this helps
It's not a problem to store this water for some time, but always, always, always double check the salinity before using. Don't just assume that because you bought it from your trusted LFS that it will be "perfect"
Listen to the Master of Disaster @Fish Fan lol he’s got some good advise there. It’s fine to store for a while however always always always hit with a refractometer before adding to tank.
Get some rest and happy reefing! That water change will be there when you’re ready!