Four years. After several years of working a series of short-term temporary jobs in the library field (anywhere from 2 weeks to one year, most of them 6 months), all the library jobs disappeared because of public sector funding cutbacks. I got a job in a bookstore, and was there for four years. Then a computers-internet-software company lured me away with a job in which I ran the company's online bookstore, and that lasted four years until the company went under. (The bookstore was making money, but was only 1% of the company; the Internet division had turned into a great big huge money pit.)
I've been with my current employer for almost three years, and intend to stay there until I retire 20 years from now.
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"Some mornings it's just not worth chewing through the leather straps." -- Emo Philips