I know everywhere it says don't keep any icing for longer than two weeks but how long will royal last? It doesn't seem that it has any thing in it that would go bad? Well maybe the wet sugar but still and how would you know if it has gone bad? Thanks.
I don't feel comfortable keeping it at room temperature for more than a week, but you can freeze it for much longer.
If it's made with meringue powder (not fresh egg whites), it's fine for 3-4 weeks as long as you store it properly, (airtight container, cool area, not in direct sunlight, etc.). After that, it can tend to start to get a slightly alcoholic smell to it. As Bonniebakes mentioned, it's better to put it in the freezer if you want to be extra safe.
Considering it's so easy and inexpensive to make, better to just throw out what you have no plans to use in the near future than to store it at room temperature anyway if you're at all uncertain about using it.