How long can mascarpone frosting be left unrefrigerated?

Depends how much sugar you put in it, amongst other things...if you post your quantities we could tell you better.

hmm. I've never actually made it! Can you recommend a recipe?

Ah OK! I don't use mascarpone, but cream cheese. I use 200g cream cheese, 1kg icing sugar, 125g very soft unsalted butter which I add at the end. A teaspoon each of vanilla extract and vanilla bean paste. This stuff never goes bad, there's so much sugar in it, and it crusts. You can store it at room temp but it gets a bit soft if its too warm. If you put it in the fridge you can "defrost" it in the microwave for a few seconds, or just wait for it to get a bit warmer if you are patient enough. 

But I think you could use the mascarpone i/o the cream cheese no problem.