it all depends: what kind of running, and what kind of biking, how long, how intense, etc? Are you running to something? Are you running from something? Is it a bear? Is the bear an average bear, or Olympian bear?

Considering the apples v. oranges nature of it anyway, the 1:3 rule of thumb (with respect to distance) has always been a decent enough calculation for me, if considering approximately equal levels of effort for either.