What is life expectancy of lung cancer patient with bone metastasis age 72 male. Had quit smoking 25 years back.

See below. It depends on several factors; pathology molecular status (egfr/alk) and other medical problems. Discuss with your oncologist.
Depends. In general stage 4 lung cancer carries a poor prognosis. Without treatment such patients tend to survive 6-12 months although some less and some more. With treatment the survival rates increase to 18-20months with a good response. A frank discussion with the patient should to determine goals is appropriate.
Six months. Bone mets mean stage IV cancer. Life expectancy is related to how rapidly the cancer is growing and what organs it is affecting. I've seen a patient with a solitary bone met appearing two years after the primary, treated with radiation, who lived five years. I've seen others who died rapidly. Six months would be a reasonable approximation, but with a significant range.