In a month of Sundays
Definition 1 of 1
Pro Tip 1/3Improbability
Use the phrase to express that an event is extremely unlikely to occur. 
He's so stubborn, he won't apologize in a month of Sundays.
Pro Tip 2/3Exaggeration
The phrase is often used for hyperbolic effect, not to be taken literally. 
You could wait in a month of Sundays, but you still wouldn't see a bus here.
Pro Tip 3/3Humorous Effect
Sometimes it's employed humorously to highlight the extent of a delay or inaction. 
I'll grow old and grey waiting for her in a month of Sundays!