How long is an eon in years?
Eon has a number of meanings. In Astronomy, an eon refers to $$1$$ billion $$(10^9)$$ years. But it also refers to a very long, unspecified period of time, or specific geologic stages of the Earth.
Since this question is posted in Astronomy, I will assume that the answer of $$eon=10^9$$ years is the most appropriate, however it is not the most common.