A magnetic viscosity study on flexible thin foils containing rare-earth alloy-powders, Sm-2(CO,Cu,Fe,Zr)(17), and Nd12Fe82B6 has been performed in order to characterize the time-dependent effects of these new and technologically relevant materials. Magnetic viscosity and irreversible susceptibility have been measured as a function of the magnetic field (0-5 T) and temperature (5-300 K) using a SQUID magnetometer. From these data, we have determined the temperature and field dependence of the fluctuation field, S-v, and the activation volume, V-ac. A different characteristic behaviour of these magnitudes is observed between both magnets, explained in terms of the dominant coercivity mechanism of each magnet, pinning and nucleation.