It varies for me. I've had pure depression which has lasted 3 months, I've had agitated depression which seems never ending. Mixed states that go for a few weeks to a few months. Pure euphoric mania has lasted a few weeks, psychotic mania has lasted a month or to.

The last major episode I had was a switch off which lasted about a month and a half. It started with a few weeks of depression (about 2) then overnight I went manic and that lasted about 2.5 weeks, only to crash for another week or two before returning to "normal." (This was all before I started treatment. Back then, even "normal" mood for me was very irritable.)