Chain "stretch" is the most important criteria (in fact, it's the only criteria) I use in determining when a chain needs replacing.
A 520 (and 525 and 530) chain has a .625 pitch. That means that the link pins are .625 apart on a new chain. That means that a section of chain 25 links long will measure 15 inches from center-to-center on a new chain. As a chain wears, it will appear to "stretch" such that a 25-link length of chain will measure longer than 15 inches when worn (of course a chain doesn't actually stretch, but a worn chain appears to have stretched when measured with a tape measure).
The problem that occurs with continuing to use a worn chain is that the pitch (distance between links) on the chain no longer matches the pitch (distance between teeth) on the sprockets.
One thing about a sprocket is that the nominal pitch changes as you move outward in sprocket diameter from the valleys between the teeth toward the tips of the teeth, because the diameter increases as you move outward but the count of teeth doesn't change, so of course the distance between the teeth increases as you move outward. And that's exactly how the chain "adjusts" for the apparent "stretch" in its length as it wears - it starts riding higher in the sprocket towards the tips of the teeth until it finds a "pitch match" instead of riding down in the valleys between the teeth where it belongs. And that radically alters the wear pattern of the sprockets, causing the teeth to wear into a "cresting wave" shape. And that undesirable wear takes place rather quickly if you don't replace a worn chain before it stretches too much. So the price you pay for not replacing a worn chain is the cost of fresh sprockets too.
So the question becomes, "what is the acceptable limit of chain stretch"?
For myself, I use the rule of "1/8 inch of stretch in 15 inches" as my own gauge of when to replace the chain, i.e., if the chain wears to the point where I can measure 1/8 inch of stretch in any 15-inch length of chain, I replace the chain (a chain typically wears unevenly, so I measure it several places along its length). If I can measure 1/8 inch of stretch anywhere along the chain, I replace it.
If you're wondering where I came up with the "1/8 inch" rule, that's the general rule bicycle shops use when determining when a chain needs replacing. Bicycles have lots of sprockets in their drive line (not just two) so it can get really expensive to replace all the sprockets when the chain is allowed to stretch too much. Also, if you don't replace worn sprockets on a bicycle even a new chain will "skip" teeth.
For a data point, my road bicycle is 27 years old, has 89,000 miles on it and is only on its third chain (I keep my chains well lubed, which effectively prevents wear inside the links, which effectively prevents chain "stretch"). None of its sprockets have never needed replacing, because I've never ridden with a stretched chain.
As long as a chain hasn't stretched beyond some predetermined limit, there's no reason to replace it, regardless of its age.