How do you store left over Royal Icing?

You can store it in the frig for a couple of week, or in the freezer. The book I have doesn't say how long it's good for in the freezer, but just says that it can stored in the freezer until you need it again --- so I'm thinking it a long time.

Hope this helps,
Tammy

you can store it in an airtight grease free bowl at room temperature for 2 weeks

I haven't worked with RI that much, but the few times I made it and stored it at room temp, it separated and got spongy. I've read that's fine, as long as you just re-mix, but it looks too nasty for me to comfortably use again

My sense is to store it in the fridge short term or the freezer long term.

I use royal icing out of the freezer all the time. I mix up a batch, color it if I know what colors I will need, portion into small containers and pop in the freezer. Take out as needed. Defrosts super quick. It does look spongy when you first take it up, but just use a small spatula or spoon to fluff it back up and you're good to go! So much easier than having to always be making a new batch, especially if you only need a little at a time.