Stage 4 lung cancer, given 12 months 18 months ago, lost heaps of weight, now large mass in kidneys, need to know how long left to take kids 4 l visit?

Cancer prognosis. Sorry to hear about the grim diagnosis. It's impossible to tell exactly high much longer without knowing the functional status of the person. 6 months is generic survival span in most cases, but it can be more or less than that depending on the aggressiveness of the cancer, etc.
Not much longer. It is always difficult (nearly impossible) to predict the inevitable: you should go now and spend as much time together as possible (for as long as possible).