Answer
20 m
Work Step by Step
The breakdown electric field of air is the voltage difference between the bolt’s ends, divided by the length of the bolt.
$$3\times10^6V/m=\frac{5\times10^7V}{L}$$
Solve for a length L of 20 m (to one significant figure).
This relatively small distance explains why a lightning strike from a cloud to the ground must propagate as a series of steps.