When Will It Happen? Not in a Month of Sundays

If you say that something will not happen in a month of Sundays, you mean that it will never happen. He is never going to get it finished. He will never run the marathon, not in a month of Sundays. You actually mean that it is not likely to happen. "In a month of Sundays" is used to emphasize that something will never happen. 

The phrase "when pigs fly" has similar meaning. it is a figure of speech (hyperbolic) that describes an impossibility or something that will never occur. "When pigs fly" is a way of saying that something will never happen. The phrase is often used for humorous effect.

Once in a blue moon is a similar phrase. However, to do something “once in a blue moon” is to do it very rarely: “That company puts on a good performance only once in a blue moon.” The phrase refers to the appearance of a second full moon within a calendar month, which actually happens about every thirty-two months.

The expression "once in a life time" stands for an experience or an opportunity that is very special because you will probably only have it once: "Going to college when you finish high school is a once-in-a-lifetime experience." The phrase "once in a lifetime" is used when you refer to very unusual or special experiences.