We assume a logarithmic normal distribution of activation volumes within an ensemble and study four different models of its average thermal relaxation. We extract from them a common rule for the time range on which logarithmic relaxation can be observed. We further show that to a very good approximation the coefficient of magnetic viscosity is ${\mathit{S}}_{\mathit{v}}$=-T/(\ensuremath{\partial}〈scrE〉/\ensuremath{\partial}H) where 〈scrE〉 is the average activation volume within the ensemble.