It is shown that phonon lifetimes can be calculated using finite periodicities and discrete frequencies without going completely to the limit of infinite periodicities or quasi-continuous frequency distributions. In applying the Golden Rule equation in time-dependent perturbation theory there is a time interval over which the transition rate is very nearly independent of time and energy level differences (or periodicity). As the periodicity of the lattice increases the time interval over which the transition rate is independent of time increases and approaches infinity in the limit of infinite periodicity. Calculations are presented.