WHEN will Phineas and Ferb get busted?

The characters Phineas and Ferb never actually get busted in the Disney animated series of the same name, as is part of the running gag of the show. Despite their outlandish inventions and adventures, somehow their parents (or adults, in general) never seem to catch them in the act and their creations just conveniently and mysteriously disappear at the end of each episode. So, to answer your question, Phineas and Ferb technically don't ever get busted in the context of the show.