Content audits are most valuable during a strategy change, or after a long, perhaps unchecked period of content creation. Why do one depends on what you want to accomplish: linkbuilding, merge content, purge pages, update blog posts, etc. Sudden changes in performance are a very good reason to conduct an audit. Bounce rate increases + avg ranking decreases + traffic decreases. If all 3 happen, audit.
It also depends on business need, and if you’re solving for now or later: small domain that isn’t growing vs. big domain worried about “site bloat” / future page speed. You might also launch new product and need the blog to rally around it.
How often you conduct audits will vary, though. It can take up to 6 months for content to mature to the point where it’s a candidate for an audit. Update content in the short term, audit content in the long term.