In my area, most timber harvest is done with mechanical harvesters, and saws ride around on the equipment, or even sit in a truck a lot. Usually saws get replaced because they were damaged, abused, or maybe the logger just wanted a newer model.
There are a lot of "arborists" crawling out of the woodwork, as out-of-work folks are looking to find new ways to earn a living...most of them will fail, as they don't really understand the work or even how to really run a business. A few of these guys are using already worn out equipment and many of the rest will never wear their saws out. When things start looking up, they'll abandon the trade and take less demanding work.
Edit: it occurs to me that I didn't even come close to answering the question.
I have a few older saws at home, including a Homelite 330 (not really much of a saw) that I personally wore out years ago: it lasted about 3 years as my only saw, and has about 100 hours on it. I think roughly equivalent to 60 cords of oak firewood. A Jonsered 670 had about 12 years and 500 hrs, still running great when I siezed it. The bottom end was fine, so I replaced the piston and cylinder.
I believe with good care, a modern nikasyl-bored chainsaw could last 1000 hours.