We have previously reported experimental studies of the magnetic aftereffect in CoCr films. In that work data were compared to phenomenological models, and it was recognized that the decay of the magnetization was driven by the demagnetizing field. The difficulty of incorporating such fields into phenomenological models, which rely on a broad distribution of relaxation times or activation energies to account for the quasilogarithmic decays, motivated the present study. Here we present a simple alternate model which does not assume a distribution of energies. Instead, we focus on the time dependence of the local magnetic field h which drives the magnetization. We approximate h by the average internal field Hi =Ha-4πM, where Ha is the applied field, M is the magnetization averaged over regions large compared to domains, and 4π is the demagnetizing factor appropriate for a planar geometry. The magnetization is found to relax quasilogarithmically with time in this model. We have also made a more detailed comparison with experimental results. We conclude that this simple model is more transparent to the essential physics of the problem than previously used models, and may be useful in analyzing decays in other materials.