I search and wasn't able to find if anyone had experience with storing AV Salinity and what the shelflife would be. The container says ready for immediate use after mixing but doesn't address anything long term so I sent an email to AV asking if I can mix and store a month's worth of saltwater and below is the response I received them. I know this is pretty standard stuff, but since I'm new to the hobby that extra peace of mind will go a long way. Hopefully, someone else can find this useful.
"It is possible to store pre-mixed salinity, however, there are a few things to keep in mind. If you store it for up to a month, I would highly recommend keeping a tight-fitting lid on the mix. When atmosphere CO2 equilibrates with the water, it can cause a shift in Ph and KH. It would be better not to keep it heated or mixing during this time, as this can cause more CO2 to get into the mix. I would also recommend checking your levels before putting it into your tank to ensure they are still in the desired range."
"It is possible to store pre-mixed salinity, however, there are a few things to keep in mind. If you store it for up to a month, I would highly recommend keeping a tight-fitting lid on the mix. When atmosphere CO2 equilibrates with the water, it can cause a shift in Ph and KH. It would be better not to keep it heated or mixing during this time, as this can cause more CO2 to get into the mix. I would also recommend checking your levels before putting it into your tank to ensure they are still in the desired range."