How long do people usually live with untreated breast cancer?

Variable: This is highly dependent on the cancer type and stage of presentation. Some aggressive forms of cancer may take over vital organs within months; other breast cancers may rarely spread to other organs and may be indolent for years and years. What remains true is that there is no benefit of "watchful waiting" when it comes to breast cancer: all else being equal, early diagnosis can save lives.

Depends.: There are some patients with breast cancer (such as dcis) can lived for many, many years with the cancer doing nothing. Some people will die very quickly. It depends on the type of cancer, the stage of the cancer when discovered, presence or absence of hormone receptors and much more. Obviously the sooner you find the cancer and start treatment the greater your chances are of being cured.

Depends on stage: This depends on the stage of the breast cancer.