What about consultants? I currently am on a "mission" of 1 year 3 months, but really the original assignment was 6 months, which has been prolonged 2 months, 3 months (really, with a change of function), and now (maybe, to be approved) until the end of the year, which would make it 1 year 8 months. But that's exceptional. And really, these were ~10 months in one function, then 5 months (for now) in another part of the company (double consultancy ... sigh). So in at least one interpretation I haven't held down a job for a year in ~4 years. Of course, I did have the same employer for 7 years. But I see my actual employer, and my manager there, maybe 2 times per year.

Because holding a single mission at a customer as a (probably expensive) consultant for a year ... I'd say my average is 8 months. My record is 2 years 7 months.