A long time, as in I haven't seen Barbara in a month of Sundays. This expression, which would literally mean thirty weeks, has been used hyperbolically since it was first recorded in 1832. One writer suggests it originally connoted a long dreary time, since games and other kinds of amusement used to be forbidden on Sunday.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.