The icing makes enough to:
Fully ice the top of two x 24cm cakes; or
Ice the top and middle of a 24cm cake; or
Ice 12 to 14 cupcakes with a double layer swirl; or
Ice 20 cupcakes if spreading it in a single layer.
I make my icing the day before I use it to give it time to "blend" in the fridge ... then I take it out of the fridge the morning I plan to use it to bring it to room temperature to spread easily.
The icing will keep in the fridge for up to a week, and freezes beautifully. If frozen, remove from the freezer the day before you plan to use it and put into the fridge, then follow the process of removing it the morning you plan to use it for easier spreading.
When I am using half the icing for a single cake topping, then I put the rest into a container and place straight into the freezer for another time.
Once on the cake or cupcakes, the icing will "hold" at room temperature for up to three days ... but if it is particularly warm (like in the middle of summer with 35+ degree days), then I put the baked goods in a sealed container in a cooler room in the home. If it is really warm and I have made the cupcakes/cake a couple of days in advance of serving them, then I put them in a sealed container in the fridge, and remove them three hours before serving to allow the cupcakes/cake to come to room temperature ... this is particularly important for butter cake / pound cake mixes which can be too firm and unappetising eaten straight from the fridge, although carrot cake style mixes are fine consumed cold straight from the fridge.
Please note when using melted white chocolate in icing, it is more temperamental that dark chocolate and can "split" if over beaten, so it is important you don't blend it for any longer than I have written in the recipe, so just follow the instructions and you will be rewarded with a delicious white chocolate icing.