And they never really tell you. You go look up the statistics and find out the five year survival rates - which are always based on what was standard of treatment then and not now. You can over think all the averages to figure out your own expiration date. You ask your doctor questions - I once asked my oncologist what my odds were (after a lot of deep thought and gut wrenching decision making) and got some nice vagueness that didn't mean squat.
Doctors seemed to be trained not to give you a solid answer. Someone posted a link to this article by a self described 36 year old non-smoking neurosurgeon who was recently given a nasty cancer diagnosis. I read it and a lot more makes sense.
The patient/doctor clarifies about what the doctors are trained to say:
"My standard pieces include “it’s a marathon, not a sprint, so get your daily rest” and “illness can drive a family apart or bring it together — be aware of each other’s needs and find extra support.”
I learned a few basic rules. Be honest about the prognosis but always leave some room for hope. Be vague but accurate: “days to a few weeks,” “weeks to a few months,” “months to a few years,” “a few years to a decade or more.” We never cite detailed statistics, and usually advise against Googling survival numbers, assuming the average patient doesn’t possess a nuanced understanding of statistics."
"In brain-cancer research, for example, while the numbers for average survival time haven’t changed much, there’s an increasingly long tail on the curve, indicating a few patients are living for years. The problem is that you can’t tell an individual patient where she is on the curve. It’s impossible, irresponsible even, to be more precise than you can be accurate."
It is true that we can't expect a doctor to pinpoint our expiration and then we cannot expect them to give an answer with any sort of specificity. But we still ask. Because we still want to know how long.