Time taken for magnetic field to permeate a spherical volume

[doppeled]

1. 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.

 

[doppeled]

It's ok, think I have solved it. Using the diffusion and convection equation and setting U=0.