a month of Sundays -- ( La semaine des quatre jeudis )

         Yes, "a month of Sundays" is a very common expression. It's an idiom meaning "a very long time."
It's interesting to note that it's not a literal calculation of time, but rather a hyperbolic way to express a lengthy period.
________________   reminded me of   [La semaine des quatre jeudis]
Traditionally, Thursday afternoons were half-days or even school-free days for French schoolchildren.
__________________________
What does haven't heard that in a coon's age mean?
              --->   a very long time
In "I haven't seen you in a coon's age",
a coon's age simply means "a very long time."
According to most sources, "coon" means "raccoon" here, and early settlers in the U.S. were under the mistaken impression that raccoons were particularly long-lived animals.