There's always a new topic (or sales ask) to cover. How do determine the team has covered one "enough" and balance the need for breadth versus depth?

This is what makes PMM so fun. And also a little chaotic. You're frequently context-shifting between strategic investments (a 12- to 18-month horizon), quarterly operational work (those "big rocks" and Tier 1 launches), and the daily/weekly/monthly execution below the scenes.

And then a competitor (or new entrant) does something you have to react to. Engage competitive skillsets!

I've found the best way to manage through this is through a few tools:

1. Clearly establish what your high-impact priorities are. And then communicate them until you're blue in the face and sick of hearing about them yourself. 
2. Understand the relative value of any activity or impact in the scope of your GTM. For example, how valuable is a datasheet vs. a battlecard vs. a microsite or landing page. How much pipeline does it influence? How often are you hearing a given objection?
3. Declare clearly what "done" looks like.

This approach can help take some of the heat off of firedrills and help put requests into perspective. To evaluate this, I still really like the Important vs. Urgent framework (also known as the Eisenhower Matrix ). 

There's always a priority shift between urgent and important tasks. What you can do is understand which a task is, and then how much time and effort you will put into it.

Also, remember: PMM, messaging, positioning, copy, and even strategy are constantly evolving things. Allow for iteration. Many times, getting to "done" is way better than getting to "right." The only person who can really tell you if you're right is the customer, anway.