Well, mine vary, but sometimes go on for months. The longest hypomania I had went on for a month and a half (once I was aware of what was going on -- for years I wasn't and can't account for time frames). My depressions are the more frequent feature and sometimes go on for months. That said, it's not all that consistent. Sometimes they are longer, sometimes shorter. Inside of the larger picture, I sometimes "flip a switch" for much shorter spans of time. I tend to think of these as cycles within cycles -- one or the other is really dominating, but there are short spells of something different. Most of the time when it's really flipping, I think it's more "mood lability". These aren't really cycles, it's just that moods can take big changes in short order.
Just my experience, but hope that's helpful!