month of Sundays
month of Sundays {n. phr.}, {informal}
A very long time. — Used for emphasis after "for" or "in" and usually with a negative verb.
I have not had devil's food cake in a month of Sundays.
When he got her first letter, he felt that he had not heard from her for a month of Sundays.
Syn.: DOG'S AGE.
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Source: A Dictionary of American Idioms
