How long did Jonah say Nineveh had before it would be overthrown?

Answer: Forty days

Jonah went into the city proclaiming, "Forty more days and Nineveh will be overturned!"

“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)