That's literally it, haha. Anytime there's a three-letter clue that involves a long stretch of time, the answer is almost always either EON or ERA. Whenever I see that, I always fill in the first E and then wait until later to figure out the other two letters!
There are the geological periods called eras (which nestle into eons). Those are quite long, by definition, for any reasonable definition of long. And so I think it’s OK.