How long is an eon in geologic time?

An eon, which is the longest unit of geologic time, is roughly one billion years long. The geologic time scale currently has four named eons: Hadeon, Archeon, Proterozoic, and Phanerozoic. Eons are further broken down into eras, which are further divided into periods. Periods are divided into epochs and epochs are divided even further into ages.