It is a state, not a milestone you announce. The signals are behavioural rather than emotional: retention flattens instead of decaying, customers return unprompted, people refer others without being asked, and demand starts outrunning your ability to serve it. Compliments are not a signal. Neither is a spike in signups from a launch post. The question is whether usage persists thirty and sixty days later.
It matters because almost every other business decision is downstream of it. Hiring salespeople, raising a round, buying ads — all of them multiply whatever is already true. Applied before fit, they multiply a leak: you spend faster to acquire customers who leave, and the numbers look like growth right up until they don't. Applied after fit, the same actions compound. Most founders who feel stuck on growth are actually stuck on fit and treating it as a marketing problem.
A concrete example: two products both sign 100 customers in a month. In the first, 22 are still active after 90 days and churn keeps climbing — customers arrive because of effort and leave because nothing holds them. In the second, 68 are still active, weekly usage per customer is rising, and nine came from referrals. Same top-line number, and only one of them is worth spending money to accelerate.