I generally use icing (stored in the fridge) for up to a week if people are going to be eating it. I admit for projects that won’t be eaten, I’ve used icing that was several weeks old. The icing is the most cooperative when it’s fresh. The color and consistency changes pretty dramatically as it sits. But usually by remixing with some stiff uncolored icing (that’s just as old), I can get the color and consistency back to the original.