If your question is related to the EON, as a time-period used in geology, it takes 500 or more millions of years.

5th

Edit:

The ancient Greeks referred to an eon as being an eternity, so by that definition, we would be in the one and only eon, but in geology, an eon amounts to about 1 billion years, making the Earth be 4.6 eons old, so, the answer would either be the first eon, or the 5th eon. Depends on how you look at it.

== In geology and paleontology, an eon is the largest division of time. Eons are then divided into eras, and eras are divided into periods. So yes, an era is shorter than an eon. More And then periods are divided into series or epochs

The longest in geological time is an Eon eg. Archeozoic Eon .

Edit: there is also the supereon, which is usually composed of 2-3 eons.