The company I was working for added a 121 certificate in 1984. I had 3600 hours, 2200 were PIC with another 500 as CFI. It was all recip but that's all they flew. Got my ATP with the checkout in the "new" equipment (it was only 40 years old.)

A more interesting way of looking at stuff might be "How long were you flying for a living before you got a 121 job?" Four and a half years.