Bouncing radar off of Venus' surface has yielded a rich body of data about the planet. Based on Venus' aphelion and Earth's perihelion, we know then that the answer is at least 38 million kilometers with a round trip of over four minutes.
Have radar reflections from Venus or any other natural solar system body been measured over distances larger than that?
Here I am primarily interested in reflections from the natural solar system body itself, so I've asked Farthest distance to a solar system object that's been determined by spacecraft transponder? separately.
For more about Venus and radar see: