This IMBC frosting is ready to use immediately. The leftovers can be kept at room temperature for up to 2 days or in the fridge for 10 days, or even frozen for up to 6 months. Just bring to room temperature and whip to restore its consistency.
Having said that – here in Israel it is way too hot in summer. So I do not keep any frosting out (just precaution). I always keep my frosting's in the fridge. And yet, in cooler months it’s possible to keep it out at room temperature. So use your own judgment. I believe it’s best to be safe than sorry