How quickly do estrogens break down in the environment?
Months to years - although the actual duration in a specific environment depends on the nature of that environment and is tied to oxygen level. Higher oxygen, faster degradation. Less oxygen, the estrogen molecules interconvert among various closely related molecules which hampers both their detection and their degradation.

For more info, please see: Environmental science: The hidden costs of flexible fertility (Nature 485, 441 (24 May 2012))  and this Science Daily article.