dogs age
dog's age or coon's age {n.}, {informal}
A very long time. Usually used after "for" or "in" with a negative.
Charlie Brown! I haven't seen you for a coon's age.
Father hasn't had a night out with the boys in a dog's age.
I waited for him for a dog's age, but he didn't come.
Syn.: MONTH OF SUNDAYS.
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Source: A Dictionary of American Idioms