My dad is dying - how much time does he have left?

My dad has just been diagnosed with Lung Cancer of both lungs (small cell cancer in one of them) and potentially it has spread to one of his kidneys. He is 70 years of age, in constant pain but will still not give up smoking and is on 30 a day! (We have tried to get him to stop but he wont). He has diabetes, has lost 15kg in the past four months and his hands and arms are starting to blacken and bruise. The specialist advised my mother that he was over the borderline for Chemotherapy however, has agreed to treat him to see if the cancer reacts positively, albeit she said it's only to buy him a bit of time! She would not give my mum any indication of the time he has left and has told my father he can take as much pain relief as he needs. I'm so upset - I feel numb and totally helpless because we have received no indication of how much time we may have. Does anybody have any experiences they can share to help and if there are any Doctors on the site, do you have any opinion in terms of expectancy? Any info at all would be a tremendous help and thanks for taking the time to read my question.