You will be fine with it over night on the counter.
I made a batch about a week ago and kept some from it to find out how long it would last. Been a little over a week and it's still looking good. There isn't anything in the recipe that would spoil that fast though. I used a recipe with the same ingredients you have listed.
Leily
I've never refrigerated my Dream Whip Icing and there was never a problem. I usually freeze the leftovers and then rewhip to use the second time.
If you are using the recipe from this site, it does not have to be refrigerated. I have used that recipe and love it!
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