How often should messaging be updated (i.e. message house) vs campaign messaging? Who should own it?

Messaging should be tested and updated on a fairly regular basis. As you conduct tests and receive feedback from sales, CS, prospects/customers, look at your messaging to see if it needs to change based on the new information.

Basically, your messaging should be updated as you see changes in the market, product, prospect/customer, and how your offering delivers value and solves problems.

Product marketing should own messaging and keep it updated, but you can then give different teams the framework to update it across their various assets and departments.