When should messaging change for a product and how should that be addressed with internal stakeholders?

Please see my previous answer on messaging refreshes. In general, it will depend on the maturity of your product or changes to product functionality, competitor, and industry landscape.

You'll want to plan for messaging updates as part of your annual Product Marketing/Marketing planning process and set expectations with your stakeholders on the frequency/cadence. 

Make sure you use your company's revenue and customer base as your sounding board here.

If your revenues are suffering, and your current customers aren't purchasing more of your product this is a sign you may want to change the messaging.

If you have a Sales enablement team, use them to make sure your global sales team is aware of changes in messaging. PMM shoudl host trainings on the updated message. NOTE: Not from a product standpoint, but from a "Why this makes life easier for customers" point of view.

Of course there will be a million opinions on your message updates. Focus on your key stakeholders only :)

Changing messaging needs to be considered very carefully. You can't change messaging every quarter because you'll never get the adoption you're striving for and if the message changes too frequently then you may also lose credibility. 

That being said, your messaging is never fully "done". You should get core messaging to a point where you feel it's ready to roll out internally/externally and then constantly look at what needs to be adjusted as time passes. Small tweaks can happen but an overall messaging structure should only be done once or twice a year max. The rule of thumb that I have followed is that your messaging should ideally last 18 months before it needs to get tuned to adjust to market movement and product evolution. 

This is why it's so critical when rolling out new messaging that you have everyone aligned especially the CEO so it can be championed in a way that it can be made successful. 

For competitive positioning, this is not the case. I think you should be flexible in how you position against key competitors for specific deals or campaigns. 

There is always room for testing new messaging through campaigns, content and a variety of different marketing inititiatives but changing the overall positioniong for a product or company should be done less frequently given the overhead involved in making it successful internally and externally. 

It should change when you position is significantly impacted by:

-the market changes (ie new tech emerges - social, mobile, AI, etc)

-your competition changes (ie your key competitor introduces low price offering)

-the macro economy changes (ie recession, inflation)

-your customer base changes (ie new target customer profile aligned with leadership)

The way it should be addressed with stakeholders is they should be part of creating it. And if they can't prioritize being part of it, maybe it's not important enough to update it. I find significant company-wide efforts (kick off, campaign, event, etc) are the best forcing function. Messaging is almost impossible to update without a compelling event because you don't get the stakeholder enagement you need to make it great and amplify it.