I have just the opposite. Lately, my mania/mixed states last far longer than pure depression. But I have had periods of uninterupted depression lasting the better part of 6 months. Most of the time I rapid cycle though. I've noticed my illness changing with time, and presenting itself in many ways over the course of my lifetime.

Just curious, what meds have you tried? For me, adding an AD (lexapro) to Zyprexa and Lithium has been helpful. Adding Lamictal to the cocktail has also been helpful. Welbutrin has helped me in the past as well. Some also find that APs are good at relieving depression (if you can handle the side effects :P)

If you haven't tried Lamictal or Welbutrin, i'd suggest starting there. Some BiPolar people are sensitive to serotonin agents. That's my advice offered from experience. I'm no expert. Take it with a grain of salt.