Time taken for magnetic field to permeate a speherical volume

For a stationary plasma of electrical conductivity 1.00 x10⁸ Ω^-1 m^-1 estimate the time taken for a magnetic field to permeate a spherical volume of 3.0 m radius.

I have been looking at the question for some time now and I am struggling with where to begin. Any help would be much appreciated.