I'm decorating my first shaped cake for my son's birthday party next month. It will be the "High Flying Witch".
Once the cake is baked and the buttercream colored, how long should I allow to do the actual decorating?
(I've piped my share of stars before. Last year I made 3 dozen cupcakes with airplanes piped on them.)
Hard to say, because it depends so much on the complexity of the cake, your skill level, and the speed you work at. To be on the safe side assume it will take longer than you expect. If it were me and I was not sure I would set aside a whole day for it, preferably the day BEFORE the cake is needed - it may not take that long, but then if it does you're okay, or if it takes even longer because something goes wrong, then you'll have time up your sleeve to get finished the next day.
I agree with Coral3, leave enough time so that you're not feeling stressed. It's hard to know exactly how long it will take but feeling rushed to get a cake finished is NOT fun I would start the day before just in case anything goes wrong and you need to redo something. The more cakes you decorate the better you'll get at knowing how much time you need, but even then it's hard. Sometime that takes one person an hour to do could take another person three hours to do. Good luck!