I can't speak for Lung Cancer-my husband has grade 4 brain tumor.
However, I can speak for dr's giving you a prognosis. In December my husband's nurse practitioner told me it would be a miracle if he lived until Easter. So, as you might imagine, I kept checking the calendar to see when Easter was. Well, it's now July and he is still alive and medically about the same as he was in December. I am realistic and I know he won't live more than 6-12 months, but until it clear that he is really at the end of his life, I am not interested in hearing another guess at how much time he has left.
So, try not to focus on the time the dr told you. NO one knows, not even the dr's how long some one has. When my father-in-law was ill with cancer his dr's told him he had maybe a year to live and died exactly a month later. While I know you want some sense of how much longer the person has, it's also hard not to feel like you are waiting for the other show to drop.