How long do you think you should wait, as the very first Head of PMM, before you hire your first team member?

It's hard to give a specific answer here because every company is unique, but here are some things to think about:

1. How experienced are you as a PMM?
   
   If you've built and managed multiple PMM teams in the past, you already have a solid understanding of the types of people you're looking for, how you plan to structure your team, what "great" looks like in every role. In that case you'll want to give yourself 2-3 months to get the lay of the land at your company, but then you might as well start building.
   
   But say this is your first time building a PMM team. I would suggest giving yourself 4-6 months (even more) before you start looking. This will give you time to roll up your sleeves and execute for a while. Learn what jobs need to be done at your org, what roles you need to fill, what "great" looks like.
   
2. What level of experience are you hiring for?
   
   If you're looking to hire senior PMMs you can likely start faster (and you should since they're hard to hire). These people will come in with their own methodologies and best practices, and if you're looking to give them a lot of autonomy, then they can likely hit the ground running. They just need time to understand the product, market, customer, etc.
   
   If you're looking to hire junior PMMs you'll want to consider waiting a bit longer until you've had a chance to do that role yourself. This allows you to carve out a role and what "great" looks like that you can then fill later.
   
3. Has anyone in your company been been doing the "job" of a PMM before you got there? For example, have there been any team members who've been owning sales enablement, competitive intelligence, sales collateral, etc. on the side?
   
   Chances are there are some people who haven't had a PMM title but have been doing some parts of PMM before you arrived. You might want to consider recruiting them right away vs taking over their job (assuming they are great at it). I've had success hiring internally for PMM roles. People already know your product, market, and customer, plus they already know everyone in the org.
   
   This has actually happened to me at Klue. In my first month I've already recruited two internal hires to the PMM team — a customer marketer and competitive enablement manager.

Get that headcount ASAP! I kid, I kid.... kind of. My selfish, well, self, wants to say ask for approval as soon as you start -- first 30 days. Hiring someone doesn't happen overnight so you want as much time as possible to get applicants. 

That said, the logical part of my brain says take some time to assess the business need and team need. I had a PMM team previously that included PMMs, PMs, Content Managers and Sales Enablement Managers. For a startup I'd bring on a content person first, assuming I can create the messaging and give direction, but need a creative person to take my (somtimes) genius ideas and turn them into thoughtful, customer-focused content that leans into the brand of the company. I've been blessed with working with many incredible content marketing gurus in my days.

There is no generic rule of thumb for this, but i'd focus on company growth as an indicator. 

If your company is growing fast and you realize that there are too many initiatives to support and that you cannot support the larger strategic company goals & objectives with just one person, then its likely time to bring another person in :)

I think this really depends on business stage and company needs.

If you are in the 0-1 phase, you can likely handle the department in single player mode while bets are still manifesting. The 0 to 1 phase involves inventing something new, testing hypotheses, taking contrarian bets, soliciting market feedback, and developing products which address a validated market need.

The subsequent phase of a business, i.e. going from 1 to 10, requires attention to a different set of tasks. Its success relies more heavily on activities such as customer management, business development, recruiting, implementation of processes and controls, automation, distribution, partnerships. This is where you'll want to go deep on building a horizontal team as it requires a different set of skills, but more importantly, a different set of interests.