Hello JL03 and welcome.  Let me say at the outset I have never had cancer but I have cared for and/or known those who have.  Lung cancer has featured with several people I have known over the years and I have found just how much individuals can vary when they have a Stage 4 lung cancer.  My son's father became ill quite quickly went downhill and died almost two months after diagnosis.  To balance that my mum's cousin has had Stage 4 lung cancer for well over a year now and is still able to get out and about although she has to take things slowly.  Others have ranged between the two.  Sorry that doesn't really help  you - as you wisely said it does seem to depend on the individual.    I think that your own observations are as much of a good guide as anything - if you feel things have started to deteriorate then of course the medics should be called in to assess what may be necessary.  I am so sorry that your mum is so unwell and hope that coming to this forum to vent your feelings will be useful to you.  Annie