The main issue with using salt quickly after mixing is making sure the salinity and ph is correct, and that the salt is fully dissolved to provide ionic balance. I think that water coming out of an R.O./D.I. filter typically has a pretty low oxygen content; not sure of that, but it seems like I heard that someplace. Anyhow, it's a good idea to aerate your newly mixed salt water, and check the ph before you add it to your tank. I've found storing mixed up water that cyano occasionally forms a little in the tub; I assume this is due to not having a sterile mixing tub. I've always used this water with no ill effects.

As far as storing R.O. water, I assume as long as your container is clean you could store it indefinitely.