This is something I am trying to figure out myself. Since I am newly diagnosed and working to get to the bottom- just found out BP 2 and PTSD. So, I never thought of myself as very depressed person before, mostly anxious, raging, and hypomanic, which tends to last a few days. I am beginning to think that my depression was very hidden even from myself. I never wanted to believe I was depressed. Just feeling moody or realistic, as I liked to think. However in reading and researching, I surely did experience depression. So, I guess tracking moods can be the only way to figure out how long the episodes last.