How fast can advanced agressive ovarian cancer without treatment cause death?

There is not really a straightforward answer to your question about how fast it takes for ovarian cancer to develop and spread. This is likely to be variable and it will depend on the individual features of the cancer. When doctors look at ovarian cancer cells under a microscope they can grade the cancer cells. The grade will give doctors some idea about how fast growing and aggressive the cancer cells look. The more like normal ovarian cells the cancer cells look, the lower the grade and the slower growing the cancer is likely to be. With high grade faster growing cancers the cells look very abnormal.