How long does the average lung cancer sufferer live without treatment?

My 89 year old great grandmother has just been diagnosed with lung cancer and she is refusing any treatment. She is in no discomfort at the moment, apparently, but gets very short of breath. She thought she had a chest infection but when antibiotics didn’t solve the problem she had more tests.

Anyway, I am just wondering how long she is likely to have left?

If anyone in the medical field can give an educated guess or if anyone has any experience that hey can draw on it would be much appreciate.