On the basis of the Maxwell model for very viscous liquid we derive equations for definition of characteristic relaxation times that are used in the integral criterion of plasticity. Relaxation times are constants for the material with current microstructure and do not depend on the features of the deformation process, but they change at the change of the main deformation mechanism of plasticity. We proposed the equations that connect the characteristic time of plastic relaxation with the scalar dislocation density and grain size of the material for dislocation plasticity and creep consequently.