It depends highly on the commercial application the saw is being operated in, as demonstrated by the posts above (and the quality of maintenance.) That being said, west coast log cutters are among the best operators as far as maintenance goes, and the lifespan of a pro quality saw here is limited to two years. Most west coast guys work a 6-7 hour day, six days a week when they have work. In the early-mid 90's, I was putting a new piston and rings in an 066 every 14-16 months and a new piston and ring in my 288s every year.
I was only running an 044 for about 18 months before it became a back-up saw, and then it only stayed in the stable another year at the most before getting sold as a project or as a firewood getter. 046s were lasting a little longer but not much. 372s start needing mounts around 18 months.
I've seen tree service companies get anywhere from a year to six years out of a pro level saw. The amount of abuse the saw receives is the mitigating factor there. A climber using an 020T/MS-200T full time will probably only get about 2-2.5 years out of one before it starts becoming a project. I've seen hook tenders get 8 years out of a climbing saw.