A very long time. It is doubtful that this expression, which dates from the early nineteenth century, was ever meant literally—that is, a period of thirty Sundays (or weeks). It first appeared in print in Frederick Marryat’s Newton Forster (1832) and was surely a cliché by the time Ogden Nash played on it in “My Dear, How Did You Ever Think up This Delicious Salad?” (1935): “The salad course nowadays seems to be a month of sundaes.” The British version, a week of Sundays, is never heard in America.