I would say when it becomes too slow for even basic tasks like browsing the web, or running an up-to-date operating system.
Today, I would say the bar is around 3000-4000 points on cpubenchmark for the cpu, 8gb of ram and an SSD.
You could definitely get a usable computer that has less. I have a Pentium II PC that works great, and can even connect to the Internet. But software today is far more bloated and inefficient than it used to be, such an old machine would be useful only if you don’t do anything computationally intensive, and don’t need to run any modern software.