My mood swings can last most of a day, but not usually longer than that.
However I can be in a depressed state for up to a month at a time.
I do have a secondary diagnosis of Major Depressive Disorder so maybe the long-term depressive cycles are related to that and not BPD.
I empathize with you, I had a really low period in November. Was contemplating things I shouldn't be contemplating.
It will pass though, hang in there.
-Hugs-