Another interesting bit of info. It is generally thought that cake mixes stay fresh longer than from scratch cakes. Now I would imagine it is because of the chemical preservatives added into cake mixes. However once frozen, cake mixes have a shorter shelf life than a frozen and thawed out from scratch cake. I have no idea why, possibly due to the chemical interaction with freezing. I actually got that from a government site.
Also, the best cake to freeze is a sheet cake as the larger surface makes for a longer freezing time without noticable deterioration. Which means that cupcakes have a shorter freezer life than regular cakes. Generally under a month is considered the best length of time to freeze most cakes with some cakes lasting up to 2 months without any deterioration. Incidentally, the deterioration is actually measured by a chemical breakdown and not so much by taste.
Regarding butter, the breakdown is more related to temperature. During the great blackout, we had both butter and shortening go rancid. Shortening will also go rancid if kept too long or at too warm a temperature and so far, I have found that the time frame for both types of icing at room temperature, to be about the same. This changes as the temperature goes up as butter has a much lower melting temperature than shortening does - I believe about 83F as opposed to 89 for shortening.
Unsalted butter has a much shorter life.
Because I worry about liability, I always buy fresh butter when I am using it for cakes where folks outside of the family, will be served, nice, eh? Haha! But I find the same thing you do, it lasts a long time.
I also keep cakes iced with Cream Cheese frosting, on the counter for our home use, mainly because that was what I always did and had no problems. However if a cake is leaving the house or serving other than family members, I refridgerate these iced cakes.
So far we are all relatively normal and well, well at least the rest of the family is, haha!
Hugs Squirrelly Cakes