They guy at the HD near me said every 3 years they get switched out, if they need it or not. The guy also said that the ones they have now, in service a year, have about 4 rentals on them each (they have 3). Mine wasn't worn out at all, still runs great, but I've only had it a season. It cost $235.
If it goes, and it is a cylinder/jug, it gives you the excuse to upgrade to the 7900!!
At the HDs near me they said the saws get sold every 2 years.