I have some royal icing that I made a year ago and it is red. It doesn't smell bad, so I imagine it is still good. I just have to stir it and stir it and stir it. My teacher in class said that as long as it smells ok, then it should be fine. You just have to gauge it I guess. If you are making cookies, I read that only fresh royal should be used. But I imagine if you are making decorations that will harden, then you can keep it for a longer period of time. The only problem I have found is that fresh royal, has a matte like finish. When kept for a while and re-constituted, it is shinier, kind of like color flow.
I would say, to be on the safe side, that it is definitely good for a month or so.