btw - how do you know the appropriate consistency of icing when you're trying to do some piping?
Depends on what you are piping. For instance, if you are using royal icing you want to have a slightly stuffer consistency compared to buttercream. The stuff that comes in those containers you showed a picture of earlier is neither buttercream or royal icing. They taste disgusting also.
When icing sugar cookies you want to use royal icing as the same goes for making gingerbread houses. This is because royal icing actually dries hard. This makes a great icing for creating display cakes in bakeries.
Buttercream doesn't dry as fast and can remain edible for a long while. Not talking months, but days. A few weeks if you are good. years if you freeze it which is what people do when they take out the top teir of their wedding cake and share it together on anniversaries.
Hnm...maybe I should request a thread dedicated to cake and sweet making advice.