This is an awesome question. Super complicated! Although i suppose the short answer would be when you are paid for the work you provide. I'm sure we have all seen things paid for that were less than the quality and caliber that someone would consider "professional".
To me it's skill level. Are the cakes produced clean, level, smooth? No tears, pokes, damage, messiness? And yes fresh and delicious too!? Of course there is always room for improvement even a Master decorator would agree to that.
For my experience (Culinary school, apprentice, pastry cook, assistant pastry chef, and then executive pastry chef) it is being granted a "Title" that makes you a professional. In culinary school you are told when you graduate you will be a chef. This is not true! I was an apprentice during school, and then a "Cook, Level 1" after school. Never was I called "Chef" until I was hired as a Pastry Chef (dept. head) in a restaurant.
So I guess my answer would be if those around you would consider your work professional? A jury of peers, so to speak? Does someone just starting out aspire to your skill level?
sorry for the longish post... I said it was complicated lol