My icing is 1/4 cup butter to 1 1/2 cups of icing sugar and a couple tablespoons of milk, so I'm thinking that with so much sugar, the answer is "almost indefinitely in the fridge."
So, as the title says, how long is it reasonable to keep the remnants of a batch? Although I've made small amounts (like the above recipe) on occasion, I'd like to make a big batch, seal it up, and use it as needed.
(I realize that it might not work as well a month or two down the road, I'll experiment with it if I decide it's ok to use.)
Best Answer
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.