do you know how a CPU is designed? it's just crazy hard to study the design of simple RISC CPUs we studied in college. And those were very simple, old processors.

A modern processor with performance that can match modern CPUs is no task for one indie dev, at all.

You need a team of professionals in the field, a huge budget and the technology to manufacture it, which you would probably end outsourcing to one of the big manufacturers anyway because it's very rare.

So the answer to your question is never, unless you're expecting low performance CPUs based on FPGAs.