Under the General Auto Forum there is a thread about tracking the nmber of rounds fired in each gun. I've done this for many years, and made the following observation.
Heavy recoiling guns, such as the .44 Magnum and .45 Colt, will have the most problems with broken parts. This simply from metal fatigue. Light loads and small calibers will usually shoot forever without major problems.
From my observations. bullets in the 200 to 350 grain weight take the heaviest toll.
From my log books, I see that as a magnum passes 5,000 rounds and approaches the 10,000 round mile post, its a good thing to inspect the gun for potential problems. So far, the most I've ever put through one gun is 16,400 rounds, this through a Ruger Blackhawk .45 Colt. The transfer bar has broken once in this gun.
In Smith & Wessons, pins have sheared off at 12,000 round mark. This, incidentally, in a Model 29 prior to having the endurance package installed.
Barrel turning in the frame occurs, especially if the barrel has been replaced or removed for any reason, such as refinishing.
Autoloaders are a whole different ball game. Their generally milder loads don't cause the wear and tear, but the violence of their actions cause sheared pins. Also, their weaker chamber areas will turn loose with heavy loads. My experience is limited with these, but in excess of 5,000 rounds fired is a good service milestone. This especially if hunting quality ammunition is used.
Bob, I had a Model 29 go past 84k once. It did need a lot of parts replaced to get that far(hands, bolt, star, springs, pins, ect.). I also had to set the barrel back and recut the forcing cone 3 times. When the top strap was cut about 2/3 through I quit shooting it.