I ordered mine 9 months before. I'm actually glad I did because I literally had to draw the cake for the pastry chef about 4 times before she understood what I wanted.
Definitely give yourself enough time to shop around. Try to find someone that can realize your dream. A good PC should be able to give you a rendering (drawing) of your cake when you tell them the sort of cake you're dreaming of.
It's a good idea to have a mental picture (or even an actual one) of the cake you're envisioning.
Congrats and good luck!