dog's 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.