How long does it take if a bullfrog tadpole already has back legs?

A week or two depending on how long ago it hatched.

It always depends on the species. A tree frog took about 5 weeks (a little over a month), and a bullfrog tadpole took 2 years! Usually, tree frogs and smaller frogs take less time while massive frogs such as bullfrogs, pacman frogs, and others take years.