By the DSM's standards, a hypomanic "episode" must last for a minimum of 4 days, a manic "episode" for at least a week, and a depressive "episode" for at least 2. These time requirements are completely arbitrary. A lot of research, actually, has discovered that the mean duration for hypomanic experiences is only 1-2 days.
My mood states vary considerably in duration, from minutes to months, so I have just decided to not use the word episode with regards to my own experience. I think that it is an ugly-sounding word anyway. Edit: I just realized that I do call my depressions that last for months "episodes" sometimes. Perhaps it is because they are lengthier, and I can easily tell which symptoms have changed from depression to depression, whereas if I tried to analyze my faster shifts so thoroughly I would just get confused.

Cycling (a much, much prettier word, IMO ), to me, refers to dramatic mood shifts, regardless of the speed at which the moods change. This, of course, leaves us with the problem of defining "dramatic"...I believe that if the mood changes are obvious enough that the person experiencing them notices them and finds them distressing, then they are "dramatic". My normal mood changes are noticeable to me, but I can let go of them, or distract myself from them. Moods from my bipolar cycling consume me.