Time of Reaction
The released neutron travels at speeds of about 10 million meters per second, or about 3% the speed of light. The characteristic time for a generation is roughly the time required to cross the diameter of the sphere of fissionable material.
A critical mass of uranium is about the size of a baseball (0.1 meters). The time, T, the neutron would take to cross the sphere is:
T = 0.1 meters ÷ 1 x 10 7 meters/second
T = 1 x 10 -8 seconds
The complete process of a bomb explosion is about 80 times this number, or about a microsecond.
This time was informally known as a 'shake' ("as fast as the shake of a lamb's tail") by the physicists at Los Alamos.