Consider whether this will be a dessert, the only dessert or a gift for your guest to take home.
Consider the type of cake, for example a serving of a Mars Bar Mud Cake need not be as large as a serving of Strawberry Torte.
Give as much notice as possible.
For Mud Cakes, Hummingbirds, Novelty and Carrot Cakes we prefer 3 days notice.
For Birthday sponges etc., usually you can order by midday today to be ready tomorrow.