I made cookies for Christmas and have a lot of the RI and the RBC left. Can I store it for another day? How and where should I store it? How long do you think it might last?

Thanks so much for all the help.!

Yes, royal can be stored a day or two. I prefer to keep mine in a glass jar, w/a tight fitting lid. I have keeps some for up to a week if I remember but it suffers some the longer you keep it. Of course, you will have to re beat it when ready to use. Make sure it doesn't have a crust on it. If it has formed a crust, just remove it & usually what's underneath is still usable.
Not sure what RBC is

I have kept royal icing for about two weeks. I put plastic wrap on top of the icing and it was in a pastic container with a lid too. I did have to mix it up again when I used but it worked just fine.

It depends on the type of RI recipe you use, the one I use plastic will ruin it. I have to store mine in a metal bowl with tin foil. I can store mine for a few day's just re whip it.