The point where you have a product a defined group of people genuinely want, evidenced by their behaviour, repeat purchase, organic word of mouth, retention, not by your conviction. PMF is felt before it is measured: demand starts pulling harder than you can push. Chasing scale before you have it just makes you lose money faster.
Benchmark. The Sean Ellis test is a useful proxy: if 40%+ of users would be 'very disappointed' without your product, you likely have fit. Below that, keep iterating before you pour on spend.