My father in law has thoracic aortic aneurysm that needs surgery? how long can he live with it untreated?

Depends on the size: The larger the size the greater the risk for rupture which has devastating impact and poor survival. Would encourage repair if a good candidate for surgery.

Difficult to say: The risk of complications from a thoracic aneurysm is variable. Many factors, including size, symptoms, BP management, ..... All influence the likelihood of having a complication. For aneurysms greater than 6 cm in diameter, rupture occurrs at 3.7% per year, rupture or dissection at 6.9% per year, death at 11.8%, and death, rupture, or dissection at 15.6% per year. This risk increases with size.