This icing freezes very well for a long time. So that's always an option too if you don't know when you'll need it next. When you're ready to use your frozen royal icing, leave it to sit at room temperature for a few hours until it's completely thawed. Again, it's likely that the liquid will separate so just make sure to give it a good mix before using. You might also need to add a little more icing sugar as defrosting can make it runnier.