No beating around the bush here, at some point even the best waxed chain will be worn out. If it elongates too much, the other expensive drivetrain components also suffer and, in the worst case, everything is ruined, apart from the shifting performance, which starts to jump with the 10 and 11 sprockets.
Changing is usually recommended from half a percent elongation onwards and I'm totally on board with that. These days, sprockets and blades are simply far too expensive to risk any wear here. Of course, it is also advisable to check the degree of wear before re-waxing a waxed chain. If this is 0.5 percent elongation, it is actually no longer worth it and the chain belongs in the bin.
With the CC-4 model from Park Tool you can measure all common chains up to the AXS models from SRAM, into which other gauges no longer fit due to the narrow design. You can see exactly how it works here in the video:
Like everything from Park Tool, this is a really good, heavy tool and made in the USA!