How often should architecture documentation be updated?
Any answer in units of time is the wrong shape. It means the update is a scheduled chore that will be skipped. It should be updated in the same change that alters the system, which only happens realistically if something fails when it is not.
From Architecture diagrams that stay true, published 9 August 2026. That article is where the reasoning behind this answer is set out, including what it does not cover.