Are you guys on the birth control pill, and skipping periods? Because that's when it happens for me - when I am on the pill, and don't take "breaks" as I am supposed to to have my period. Then I will suddenly get it, when I have skipped too many times for my body to deal, and then I can have it for like two weeks.

But if it's not that, I guess it is stress or major life changes. Mine has never been consistent either, though, it has varied around a week in each direction - this goes for the months I am off the pill and the years before I started it. I have never been one of those with a predictable cycle - so that's not something to worry about, I think.