A realistic evolutionary model must match the generation-to-generation time scale being modeled and employ rates that are representative on this time scale. Time in a Brownian diffusion evolutionary model is time in generations, a requirement often overlooked or ignored. Forward-modeling of Brownian diffusion shows that the time span of phylogeny’s influence on body weight in mammalian carnivores and ungulates lasts no more than about 10,000 generations, or some 57,000 years, less than one tenth of one percent of the combined group’s evolutionary history. Statistical dependence on phylogeny does not extend automatically to any or all characteristics of interest, and dependence on phylogeny may never last long for the morphological and life history traits we most often study in comparative biology.