This is a hard question because I've found that even the nature of product marketing and the title "PMM" means different things at different companies. Generally, however, I'd look at the framework for seniority against the following dimensions:
- Product coverage: Early in your career you may manage a few products, with "few" defined relative to your company's portfolio. As seniority grows, you'll stop owning products but start owning categories of products. The idea here is that at a more senior level you'll be trusted to not only tell the story for the products but also uplevel them into meaningful value by audience, by use case, or by some other dimension.
- Scope of responsibility: Often your job starts with launch (the bread and butter of product marketing!). As you get more senior it will grow and can take on dimensions of sales enablement, campaign design, thought leadership, analyst relations, PR, public evangelism and more.
- Stakeholders: Typically the first level of PMM will interact closely with a few PMs and probably some members of associated teams across marketing and potentially customer engineers / solutions consultants. Basically folks who have an intimate interest in your products alone. As you get more senior, your relationships will both elevate in level (ex. from PMs to PM Directors to VP PM) as well as a diversity of teams (ex. from Marketing + PM to add on Sales, Sales Enablement, execs, Legal, Analyst Relations, even outside agencies and influencers). A senior PMM deals with decisions that are more complex with larger consequences.
- Visibility: This means internal visibility - how well people are observing your work - as well as external visibility - how well people know you. More junior PMMs often work on projects that are critical but less visible, meaning that you have often more room for leniency if something doesn't go as planned. This is a good thing. As you move up to a senior PMM, the projects you own often have executive interest and that will increase the complexity of your decision-making process as well as the precision with which you need to execute. Senior PMMs also frequently meet top customers and speak publicly at events.
The big thing is to keep in mind what you are learning. As long as you are learning and stretching, and demonstrating that to your leadership and those who influence the advancement process at your company, you will grow into a senior PMM or a more senior PMM. I look for evidence of working at the next level, and for a sustained period of time, as I've worked with team members to grow in their careers.