I have never worried about my icing crusting before, but I do use a crusting recipe,
Should I put it in the fridge to crust, or leave it on the counter?
Counter like the other have said. It will not crust without air. Putting it in the fridge will prevent crusting. Ever leave a bowl of buttercream uncovered? The top will crust where the air hits it. Putting it in the fridge will firm it up but it doesn't crust w/o being exposed to air.
I never refrigerate a cake. I did once and it too just short of forever to crust once I removed it from the fridge.
Thanks guys, it worked!