I have had two identical office chairs now for over 8 years.... Frankly last year was when we should have replaced them, but they were very sturdy and comfortable for around 6.5 years.
I'm probably going to have to make a thread asking people for similar ergonomics to mine because the reason they haven't bee replaced is because we haven't found a similar enough seat.
Gone through a handful of crappy chairs in a matter of 1-2 years though, price premiums are definitely worth it for most furniture, I think.