The Cretaceous–Paleogene (K–Pg) extinction event did occur over a duration of at leaste a few millenia; there are debates over exactly when it ended, but the start is widely held to correspond with a thin layer of iridium-containing sediment which is not generally found in the Earth’s crust. Although the event probably did cause a world wide winter due to dust thrown up into the Earth’s atmosphere that may have lasted several years or possibly a decade, the real cause of mass extinction was the widespread effects on photosynthetic life and a resulting reshuffling which favored species and clades which were more adaptable. This occurred across all kingdoms but particularly affected the megafauna which predominately consisted of dinosaurs which were consisted of both reptilian species and pre-avian species. The smaller and more adaptable aves and reptiles survived but the class of warm-blooded creatures in the class Mammalia suddenly came into their own in the scavenging environment of post-apocalyptic new era and competed and grew into existing and new environmental niches including flying animals (bats), Pleistocene and Eocene terrestrial megafauna (unglate herbivores; canine-, feline-, and hyena-like obligate carnivores, and ursines), and the variety of marine mammals which came from diverse paths of terrestrial mammals. Euarchontoglires, including primates, stayed small until much later.
So while the climate effects of the initial impact only lasted for a few years or perhaps a couple of decades, the downstream effects on evolutionary fitness on both microfauna and macrofauna due to the impact on photosynthetic flora had long ranging impact upon all life on Earth for thousands of years and altered the course of evolutionary development by disrupting the existing homeostatis. And we are statistically due (according to some, overdue) for another impact of comparable size and destructive capability. Consider that next time you hear politicians arguing to spend money on weapons and wars, or whether to allocate a paltry few billion dollars to planetary exploration or solar orbiting satellite mission.