From the very beginning and throughout the development process.
This is very much related to the previous question. I will try to bring to life how the process works on the Square teams.
As early as possible! I pull in product marketing as soon I have a good understanding of the problem statement and its benefits to customers. Typically the artifact we'll share at this point is a draft of a PR/FAQ. If you're not familiar with this term, it's a practice taken from Amazon, where they start feature or product development by authoring a strawman press release (PR) that a customer could conceivably read and understand the value proposition of the product.
Having PMM's eyes on this early ensures that PMs aren't too inward-facing or talking about too much technical mumbo-jumbo that obscures the nature of the problem. Plus, it gives PMM a good sense of the assets they'll need to update or create in order to support the feature when it gets released.
The FAQ portion of the document is something we'll add to as development progresses. But it's important to get PMM's feedback on the PR portion as soon as practical.