The moment when your product clearly solves a problem people will pay to solve, repeatedly.
PMF is a binary: you have it or you don't. It's not '80% of the way there.' You know you have PMF when customers retain and expand without convincing, when they refer friends without prompting, when you can't keep up with demand. Until then, you're still searching. Companies often raise on 'traction' that isn't PMF yet and then get punished later.
Trying to scale a business without PMF is like pouring water into a bucket with a hole. Find PMF first; then add fuel. This sequencing is one of the most violated rules in startup-land.