Keep it aerated with a pump or air stone and it should be ok for a while. Stored salt water can accumulate ammonia over time so I wouldn't keep it too long. I use to put in a small liverock when I kept salt water this way and it seemed to work pretty well.
I prefer to keep a trash can full of RO water then mix in the salt a few days before the water change. I keep it in a 30 gal trash can, and I do 30 gal water changes, so it works out well. If there was ever a earthquake, I have 30 gal of water as long as I haven't added the salt yet. I also add buffer to the RO water as RO/DI strips all the buffer from the water. This allows the water to stay fresh longer and keeps the PH up, and when I mix in Instant Ocean, my alkalinity will be at the right level. Pure RO/DO water will react with whatever is in the air, nitrogen, CO2, etc, so adding some buffer will reduce these reactions as the water can react with the buffer instead.
This has worked very well for me and allows me to install a float switch on the trash can. If I take some water out for make-up or whatever reason, the trash can will fill back up. When I mix in the salt, I just turn off a valve to the trash can so I don't have to worry about the float adding more H2O. When I had a smaller tank, I just emptied the fresh RO into another container and mixed up the salt. Then the float in the trash can replaced what I removed. This worked a lot better than storing Salt Water.
Hope that helped.
