Distribution of τ for lensing by selected ﹩n=1.5﹩ polytrope clouds. Curves for different cloud models are labeled according to Table 1. The timescale τ is defined to be the time during which the magnification ﹩A> A_{t}=1.1﹩. The rate of events with timescales larger than ﹩\tau _{0}﹩ is shown, for detection efficiency function ﹩\phi _{\tau }=1﹩. Timescales of a few days are typical; for model F, for example, 50% of events have ﹩\tau < 3﹩ days. The solid line labeled ﹩\phi _{\tau }﹩ shows the detection efficiency function used in the MACHO event rate calculations in Figs. 7 and 8.