My sister had a small cell lung cancer which was very fast growing and aggressive. When it was found (she had been having back pain which was getting to be excruciating when the cause was finally discovered) it was Stage IV, which had metastasized, of course. Her Dr. told her she might have had it 3 months and at the same time telling her it was very treatable, if not curable. (I will say that she may not have been completely upfront about those facts, because she wouldn't tell us the very worst news/chances. My mom found out later that her Dr told her she had 6-9 months)
Her chemo was extremely aggressive, and it helped for 3 months when she all that was left was a small speck showing on the PET scan. 2 months later it was all over her again and it seemed like a an out of control freight train - nothing could stop it. 6 months was all she had from diagnosis to death.