There are a few SEO-related things that we only need to review perhaps once a year, such as reassessing the titles and descriptions and mapping out the internal linking. If you never get around to these, it’s not the end of the world. At the other extreme, there are what should be almost weekly tasks, including checking into Google Search Console, and monitoring positions for key search terms.
In the middle are quite a few jobs that get neglected: potential problems if you don’t do them, but never quite important enough to get to the top of the to-do list. It’s these which I think smarter people put on a 3-monthly schedule.
The first is taking an in-depth look at Google Analytics – not just the headline traffic, but the pages getting viewed, the sources of traffic, the geographical location of visitors and the tech being used to view the site. You don’t really want to identify long term trends two years after they became a really significant factor.
A second task is a full site crawl to identify broken links, missing tracking codes, accidental noindex tags and the like. As frequently mentioned here, the pro tool of choice is Screaming Frog SEO Spider. Less costly or free workarounds are available.
Finally, check the robots.txt file and XML sitemap(s). Are they up to date and doing what you want them to do? It’s amazing how often these get inadvertently amended or automatic updates fail.
Get those items covered if your website health matters to you, and you’ll sleep easier.