How long does butter cream icing keep?
Well you're kinda right about it lasting a long time due to the sugar content.  That will help the preserving process to the extreme.  However, to be pratical you should be looking at how long the butter fat will survive in the fridge before it starts to take on funky flavours and loses it's moisture.

You'll be fine in an air tight tub for a month or so but then flavour will suffer.  If you freeze it (it can be done but the quality will really suffer later) you can get away with 4-6 months before freezer burn kills it.

Honestly, I'd just do smaller batches and enjoy the higher quality product.  For what you save on time doing the larger batch, you'll give up on taste and quality later.