At what stage should product marketing collaborate with product design on the design process?

At the stage before anything is handed over to eng for design. At Atlassian, we refer to this as the "Explore" phase–"Wonder" and "Make" come before and after, respectively. 

When you do this, be clear on the role PMM is playing and the type of feedback the design wants and needs to move forward. In my opinion, that feedback should not be focused on visual design.

Design and Marketing should ideally go hand-in-hand. 

If the Design team is very mature, experts in the area, and functioning on a world-class level, then Product Marketing's input/review can be focused on later stage designs, possibly as key stakeholder sign-off.

In younger organizations where Design is still a developing area, Product Marketing can and should add value to every stage of Design. In my experience, it can be helpful to frame input in terms of PMM competencies (e.g., research, interviews, surveys, competitive analysis, market data) to avoid being perceived as over-steppping bounds.

If I understand your question correctly, constantly. No matter if you're the lead product marketer for an enterprise product, SMB, or consumer; whether you have a product-led go-to-market or a traditional enterprise sale; product marketing and design are inextricable partners in a product's success.

Just like pricing is marketing, product design is also marketing. It's how you (your product) explain your value, function, and potential use cases to the end user. If you need to show how your product can be more efficient than a competitor, work out a better flow with UX. If you need to understand exactly why a competitor is perceived as more usable or scalable, ask UX to break the task flows down for you. 

Practically, you want your website to reflect the absolute best of your product in action. If your product team and your brand/web/design team aren't in sync, you're going to be setting the wrong expectations. 

Beyond that, at least half of a product design's team purpose is to deeply understand your end users. Their challenges, priorities, mental models, hopes, and dreams. That's persona gold, whether your end user is a buyer, an influencer, or both. And it's a win-win relationship; you can learn more about your end user in 30 minutes than a month of research, and your design team gets equal insights into the buying process, where friction exists, what other use cases exist in the market that might be competing for attention, or adding friction to the process.