I would say almost from day 1. Even at a lower price, it's an important step to validate an idea. When people need to put their credit card in and give you money, it's another level of commitment that I don't think you will have with a free plan.

It is a level of commitment from the user but also from you. If you're getting paid, it's expected that, for example, if a service is down/not working, you need to fix it.

For many years, I've tried to put side projects out and was never to keep up with them. Only on my latest project (https://isdown.app) did I put the payments. And from the moment people started paying, I felt I was in debt to them, creating the extra motivation to keep going.