An observational study out today on NPR linked higher soy consumption to a longer life after breast cancer, especially for women with hormone receptor negative breast cancers. This study was based off the Breast Cancer Family Registry of 6,235 women with breast cancer in the United States and Canada. The women completed food-frequency questionnaires, which is not the "gold standard" of medical research. However, researchers could estimate how much soy they consumed. In this study, the most benefit was seen with greater than 1.4 mg/day of soy isoflavones.
Note: it's best to consume organic, non-genetically modified soy products. Also, fermented soy is traditionally eaten in Asian cultures, which has many health benefits in part to the probiotics naturally creating when fermenting foods.
The observational study from the FASEB Journal, April 2016 vol. 30 no. 1 Supplement: 902.8http://www.fasebj.org/content/30/1_Supplement/902.8.short
Info on fermented soy: