Howdy
It is a little bit disingenuous to state that revolvers go back to 1836. Yes, that is when Sam Colt introduced the Paterson Model, but it was a far cry from the later Cap & Ball revolvers that followed it, and a further cry from the Single Action Army which made its debut in 1873.
Anyway, this is the part that breaks most often in a Single Action Army. The split trigger/bolt spring. This one broke in one of my 2nd Generation Colts quite a few years ago. I bought the gun used around 2000, it shipped from the factory in 1968, so I have no idea how many rounds were put through it in that time. I have probably put a few thousand rounds through it over the years, but certainly not an astronomical amount. I can tell you that I have had occasion to replace a few of these springs for friends over the years.
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These are the parts of the lockwork of a Colt. The parts in most Italian replicas are pretty much the same. They may not be a drop in fit, but they are pretty much the same. The next most common breakage in a SAA is the hand spring. The hand is the part to the right of the hammer. The spring is peened into a slot in the hand, and it most commonly breaks right at the sharp curve where it emerges from the slot. The tight curve puts the most stress on the part. I have replaced a few of these over the years, once I replaced one for a friend at a CAS match during the lunch break. In a pinch, this spring can be replaced with a bobby-pin.
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I have a few 1st Gen Colts. None go back to 1873, but they are all over 100 years old. These are the parts to the lockwork of one of my 1st Gens, this one happens to be a Bisley Model. This one left the factory in 1909. Again, no idea how many rounds have been fired through it over the years, but everything still works fine, knock on wood.
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But the OP's question was not about the SAA or its Italian replicas, he asked about the longevity of "a modern single action (example being a Ruger Vaquero)".
Way back when Ruger introduced his first Single Action Revolver, the Single Six, around 1954 if memory serves, the biggest change he made was replacing the old 'leaf' type flat springs with coil springs. Because of their geometry, leaf springs are more prone to failure than coil springs. It's not that coil springs are indestructable, but the total amount of compression of a coil spring is divided by the number of coils, so each coil deflects far less than the 'bendy part' of a leaf spring. That's why coil springs are almost indestructable.
This is a Three Screw Ruger Blackhawk that left the factory in 1958. This was before Ruger began putting Transfer Bars into his revolver. Not a leaf spring in sight, nothing but coil springs. Using coil springs upped the parts count from the SAA, in the SAA each leaf spring was formed to directly interact with the part it was moving. Coil springs required a separate plunger for each spring, to impart the spring force to the parts. But coil springs made these revolvers pretty indestructable.
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Here is a modern Blackhawk disassembled. I actually bought this revolver back in 1975, but the internals of modern Ruger Single Action revolvers have not changed in all that time. Again, no leaf springs, nothing but coil springs. About the only thing I ever hear about failing in a modern Ruger is the Transfer Bar (the long, thin, vertical part attached to the trigger) can break. This usually only happens to CAS shooters who have put thousands of rounds through their revolvers before the Transfer Bar breaks, it is relatively unusual, but not super rare for one to break. In addtion to this Blackhawk, I have several Vaqueros, both 'original models' and New Vaqueros. It is often said they are built like tanks, which is true. None of mine have ever had any problems.
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Anyway, I have no experience with Glocks, or the Beretta mentioned, so I cannot comment on them. I do have a few 1911s, one was made in 1916 yes, 1916. So far it is still going strong, knock on wood.
But that is what I can tell you about the longevity of some of the most common types of single action revolvers.