"A Month of Sundays" is a colloquial expression in American English denoting an indefinitely long period of time. To get a month's worth of of Sundays (taking the idiom literally) you have to go at least 30 weeks, so if you only worked on Sundays, say, you you'd get the equivalent of a month's worth of things done in a span of 6 or 7 months. Or maybe 8-9 months of an academic calendar, since 30 weeks is two fifteen-week semesters.
I guess it also depends on whether you define a "Sunday" as 24 hours or 2-3 good hours of actual writing. A month of writing is really only about 40-50 hours.
Suppose I have five chapters of a book to write. I think I could write each one in a month of Sundays, in other words, a month put together out of scattered days from several months. My first month will be May and June--what's left of this May and the entire month of June. The next month will be will July and August--from July to the beginning of the school year in late August. Next will be September through December. That's four months, I know, but only the last part of December has no school or commuting. Next will be January and February. Once again, every "month" most contain some period of time when I'm not teaching, in this case the part of January before classes start. Finally, month 5 will be March-May. Three months, with time not teaching at the end of May. So five months of work can be accomplished in 13 months of real time. That would be quite ambitious; I'm sure I won't stick to that. The problem doesn't seem to be time as much as energy.