How long it takes for your product to achieve PMF?

There are projects on my account that immediately after launch bring revenue to the client and end user benefit/convenient service. But there are projects with the aim of long-term return, we cannot fully understand what problem of the client we are solving, as the set of these problems may be different. Realizing part of the product, we feel the market and the client, whether he is ready to pay for it and the development is worth it.