At the most, within a few weeks. Though mine went undiagnosed.
The onset of PPD can occur up to a year postpartum though, I believe.
I noticed it almost immediately but LO was a preemie so I was stressed too.
But I didn't realize I had PPP until a few months later after it got really bad.
About 2 weeks, but it got way worse at about 2 months.
2 weeks max, maybe earlier. It came extra early with my second but that was because she was life lined to a NICU and they thought she was going to die after being born. So the dr immediately started me on anti depressants given my previous history with depression and PPD.
I was around 5mos pp when I realized something was amiss
I felt "off" right away. But I knew for sure I had PPD at about 2 weeks pp.
When she came home from nicu at 6 weeks
I was in denial until about 2 month PP and I was out of the depression at 6 months PP.