Once they are put on a morphine drip, how much longer is a lung cancer patient likely to survive?

Some of you may know that in July 2011 my Grandmother was diagnosed with lung cancer and told that she probably wouldn’t see Christmas. Over a year later she is still with us but is now going down hill fast, in a great deal of pain and hardly conscious. The nurses have now put her on a morphine drip, which, I am told means the end is close. I’m guessing we are looking at days rather than weeks but if any of you can share your experiences I would be really grateful.