An era is not a defined number of years. Rather, it is a period of time marked by certain characteristics, such as historical events. In geology, an era is composed of periods.
According to the Encyclopedia Britannica, geological eras encompass millions of years. The main eras, according to geologists, are the Paleozoic Era, the Mesozoic Era and the Cenozoic Era. The Paleozoic Era, for instance, was approximately 291 million years long, while the Mesozoic Era lasted about 185.5 million years. The Cenozoic Era has been about 65.5 million years long so far as it runs to the present day.
Historical eras are defined by certain historical events or a distinctive period of time. For instance, the Elizabethan era is defined by the reign of Queen Elizabeth the First, who was the Queen of England from 1558 to 1603.