All 3 Uses

eon
in
The Sea of Monsters
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I mean, you'd think the titan lord would've learned his lesson eons ago when he was over-thrown by the gods.†
Chpt 5 *
She explained that there was only one immortal winged horse named Pegasus, who still wandered free somewhere in the skies, but over the eons he'd sired a lot of children, none quite so fast or heroic, but all named after the first and greatest.†
Chpt 6
My dear young cousin, if there's one thing I've learned over the eons, it's that you can't give up on your family, no matter how tempting they make it.†
Chpt 7

Definitions:

(1) (eon)
the longest commonly used division of geological time

or less formally: an immeasurably long period of time
(2) (meaning too rare to warrant focus)