There is no set answer to this question, as there are so many variables in play. Some things doctors would consider in making their prognosis:
1) How strong is the person physically and how healthy (a smoker or someone who is obese would usually have a shorter timeline)
2) What kind of cancer is it? With early detection, prostate cancer has a 90% survival rating. With pancreatic cancer it's only 5%.
3) How advanced is the cancer? With early detection almost anything is possible. Sometimes people ignore symptoms and aren't diagnosed until it's too late to do anything about it.
4) Age of the patient - those who are very young or very old usually have undeveloped or compromised immune systems, and are generally less able to withstand the disease.
5) Response to treatment - everyone's body chemistry is different, and in some cases chemotherapy and radiation are extremely effective, and in others there is almost no effect
6) Location of cancer - this determines whether or not surgery is an option. Brain cancer is very difficult to operate on, for example.