Hi, 

My nan didn't have lung cancer but she had breast cancer, end stage copd and emphysema. Her sister my aunt also died of lung cancer. 

I'm not a medical Professional and can only say from personal experience that it was an infection that they died from not the cancer itself. My aunt survived 6months after diagnosis, but family members would not listen when I told them it sounded like she had pneumonia so no antibiotics where given, she died of pneumonia. Prompt treatment of infections is very important, watch out for signs, they stop eating, drinking, have frequent accidents (bowel and urinary), and delirium. 

The most you can do is just be there for them, my nan lived with me and she died from end stage copd, for the last month of her life I knew the end was near, she had an infection that just wouldn't clear, I think when you are really close to someone you just know, 

I'm not going to lie and say it was easy, the last 5days of my nans life where probably the worst 5 days of mine, but I just sat with her, helped her eat and drink when she felt able to, and when she went into a deep sleep, I wet her mouth so it didn't become to dry and moisturised her lips and carried on talking to her as of she was talking back. That's all we can do. 

Trust me you will have a great care team around you, you won't have to do it alone. 

As for life expectancy I couldn't say as each person is different, my aunt was 6months, my nan went on to live for two more years after she was put on palliative care at home.

I'm really sorry your going through this, try to be strong, I know everyone says this but my nan really struggled in the end for her every breath, and as brutily heartbreaking as it was for me to let her go, I knew she didn't have to suffer anymore. So I hope when the time comes, you can also take comfort in that he won't be suffering anymore. Death is not easy, I still googled my nans symptoms and conditions for weeks after she had died, searching and searching to see if I could of saved her, even after death when there was no possibility, my brain just couldn't comprehend that this time I hadn't been able to. Make sure you look after yourself as much as him. 

Kind regards 

Cassandra.