How often should a fairly mature product re-assess its messaging?
Overall, you should at least look at it annually. Based on the progress we have made, the market shifts, etc, is this still the best way to articulate the value? Is it still working with field? Sometimes you need to adjust messaging to get sellers excited to go pitch it again.
You also need to evolve your messaging on a mature product as you go into new markets. A mature product probably started in a few key verticals but hopefully you figure out how to evolve into new verts. You will need to evolve the messaging to enter those markets.
Whether your product is new or fairly mature, you should always have your ear to the ground, listening to market pivots and changes in how customers talk about your solution, the value it delivers, and the challenges with prospects.
Even if your product is mature and your messaging was solid 6 months ago, changes in the market or new ways your product delivers value may require new messaging.
However, you don't want to drastically change your messaging too frequently. Allow enough time to collect feedback and find what resonates and doesn't. Collect the input and modify the messaging if something doesn't resonate well with your ICP.