To test consistency of frosting:
Scoop some up with a knife and try to shake it off. If it falls off without shaking, it's too thin and needs more powdered sugar. If it doesn't shake off on the first shake, it's too thick and needs more water.
This is actually a double recipe, and I used about 3/4 of it to frosting my cupcakes. This would be equal to between 2 and 3 cans of store-bought icing.