How viscous is the Earth's mantle?
The mantle viscosity is likely to be non-linear, e.g., it could be as low as $10^{18} \textrm{Pa}\cdot\rm s$ (over shorter time scales) or as high as $10^{21} \textrm{Pa}\cdot\rm s$ (over longer time scales). In any case the values reported in the literature are somewhere between $10^{18}-10^{21} \textrm{Pa}\cdot\rm s$ and these are based on studies from earthquakes, glacial rebound etc. So you can calculate the relaxation time(s) using these two end member values. 

E.g., if you assume the average i.e., $10^{19.5} \textrm{Pa}\cdot\rm s$ then we get a relaxation time of $10^{19.5}/150\cdot10^9$ (viscosity/mu) which is $2.1082\cdot10^8$ seconds or ~$6.6$ years.