How many days did Jonah announce before Nineveh would be overturned?

Answer: 40

Jonah proclaimed, "Forty more days and Nineveh will be overturned!" The Ninevites believed God and proclaimed a fast.

“On the first day of his journey, Jonah set out into the city and proclaimed, “Forty more days and Nineveh will be overturned!””— Jonah 3:4 (BSB)