Cake – wrong with the icing

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.

You have three options:

1. Prepare for drippage. If you're doing a lot of small items in particular, you set up a wire rack over a sheet pan, and just let it drip down. For the small items, you can dip into the glaze, then invert and place on the rack, like you would glaze a donut.

2. Add a barrier. Make a thicker icing that won't flow, and pipe right at the edge to give it a little bit of containment, then flood the top with the thinner icing. You'll need the cake nearly level for this to work well.

3. Pipe it on. You need an icing that's just thinned to the point where it will slump and flow back together, but not so thin it'll flow off. Use a piping bag (or plastic bag with the corner cut off) to make lines close enough together so the icing forms a solid covering. You may want to start at the outer edge and make a ring like #2, then work inward. This can work for more uneven cakes, as it doesn't need to be perfectly level, but it's more effort to get the consistency right.

Exact technique and consistency is going to depend on how thick (depth) you want the icing. It's always easier to thin it (doesn't take much added liquid) than to stiffen it back up.