A long time. In science it’s 1 billion years.
For most people, an eon is just a non-specific ‘long period of time’, and that’s the way it’s been for eons and eons. Well, as far back as 1642 at least.
Definition of eon/aeon
1: an immeasurably or indefinitely long period of time : AGE…
Scientists, however, like to be very precise about things so they adopted the name eon to describe a specific unit of time.
Eon, Long span of geologic time. In formal usage, eons are the longest portions of geologic time (eras are the second-longest). … Less formally, eon often refers to a span of one billion years.
Encyclopedia Britannica further explains that “Three eons are recognized: the Phanerozoic Eon (dating from the present back to the beginning of the Cambrian Period), the Proterozoic Eon, and the Archean Eon.” Another Britannica article explains that there is another called the Hadean Eon, so I guess there are 4.
So what are the other long units of time called?
In order from longest to shortest here is a list of geologic time units.
- Eon (e.g. Proterozoic) – 4 total, half a billion years or more
- Era (e.g. Mesozoic or Cenozoic) – 10 defined, several hundred million years
- Period (e.g. Jurassic or Cambrian) – 22 defined, tens to ~one hundred million years
- Epoch (e.g. Holocene or Early Jurassic) – 34 defined, tens of millions of years
- Age (e.g. Calabrian) – 99 defined, millions of years
Geologic ages are further divided into Chrons which represent the time between geomagnetic reversals.