What is the known maximum ranging distance of a bat's sonar?
This paper finds some species can detect as far as 67 meters, but the range varies between species.

Note that the bats can actively change their range of detection and trade off range for resolution (low range with high speed resolution for hunting in closed, cluttered spaces or high range with low speed resolution for hunting in open spaces), as explained in Wikipedia's article on Animal echolocation.

Cool stuff.