I've been through this with a lot of clients and there is a very simple answer:

Essentially you are always ready. Iterating and validating go hand in hand. You can almost turn this on its head and ask "When should I stop validating and continue iterating?"

Iterating without validation is a risk because you are working in isolation and don't know whether your assumptions are correct. So validation should be built in as part of each and every iteration. That includes pre-MVP, when your startup is still just an idea in your head. In the early days that might mean discussing design drafts over coffee, in the later stages that might mean looking at analytics results and A/B tests.

Think of it as a continuous, ongoing process, a conversation with your target market, rather than a stop/go affair where you disappear into your office for a few weeks and then re-emerge, ready to validate.

Each stage continues until you start to hear the same answers and are not learning anything new. Then it's time to ask new questions and for that you most likely need to move on in your product development cycle.

It would be great to hear more about your product, feel free to get in touch.

Best of luck - Nils