How many years did the Proterozoic Eon last?
The Proterozoic Eon extended from 2500 Ma to 542.0±1.0 Ma (million years ago) so lasted about 2 billion years.
The beginning of the Proterozoic is usually marked by the accumulation of oxygen in the atmosphere. This was the result of photosynthesizing bacteria that took in CO2 and released oxygen as a by-product.
The end of the Proterozoic is marked by the explosion of multi-cellular eukaryotic life (like trilobites, clams, etc) which was also the beginning of the Palaeozoic Era and Cambrian Period.