What length of time does the geological time scale cover?

Since the paleozoic the Cretaceous and Carboniferous periods were the longest, each about 80 million years. The Neogene (Cenozoic period preceding the current Quaternary period) was only 22 million years. Periods are marked by significant climate or environmental change, including mass extinctions and the proliferation of new species.

Everything before the Vendian is lumped into the Proterozoic period, extending back about 1.9 billion years. Geologists may divide that up further as they determine periods of significant change recorded in ancient rock.