How long does post-partum depression last? Can it last indefinitely?

Depends, and yes. Each woman is different. Most post-partum depression lasts 4-6 months, and resolves once the woman's hormones settle back into their pre-pregnancy state. For some women, especially those who have a family or personal history of depression or mental illness, the mood disorder can persist. Medicine is a viable treatment option for both temporary and longer term depression.