In this paper, we assess the frequency dependent rheological data of several different linear and short-chain branched metallocene catalyzed polyethylenes with respect to the description of the viscosity functions. The aim is to derive material specific relaxation time ${lambda}$ from the fit. This characteristic relaxation time follows a similar dependence to the molar mass $M_w$ as the zero shear-rate viscosity, but slightly depends on both molar mass distribution and comonomer content. The transition between the shear-thinning region and the terminal regime is broader for mHDPE than for mLLDPE and widens with increasing molar mass $M_w$. This finding is explained by the differences in the plateau modulus $G_N^0$ and the increase of the normalized terminal relaxation time in comparison to the characteristic relaxation time, respectively.