When did the bullfrog evolve?

Well the question is difficult to answer, because they evolved over many millions of years and are still evolving. Evolution is a never ending process. However, amphibians clearly form from "lobe finned" fish about between 330-360 million year's ago. Amphibians then radiated into many successful branches including reptiles which then continued developing into lizards, mammals, testudines, dinosaurs, etc. Frog-like animals specifically seem to go down their own path in the early Triassic (175-199 million years ago), and Vieraella, the first "true frog" of the form we would recognize today as a "frog" is found later in the early to mid Triassic (150-175 MYA). By then most forms of amphibian no longer existed, and likewise all early forms of frog have also evolved into what we see today.