Hey purplebutterfly...

Well, I looked into it, and there really isn't an answer on the internet. I think it's dependent on a lot of things... and there are 3 different types of postpartum depression as well!

I believe I read it should only (normally) last a few weeks afterwards... but if you're already depressed, or have coexisting medical/psychological conditions already... I'd assume it could last much longer than that.

Hope that helps!