Which one? I've had 39. (That I can remember. )
The longest was 6 years, but… it was seasonal, so it's not the same as 6 real years… Shortest? No idea. Couldn't even give you an average, as that would require record-keeping, remembering and/or having a sense of time. None of which are in my skill set.
Never had a job with a chair. Though I have lasted long enough that I could have sat down for a moment had there been one.