In the UK, the Government Digital Services (GDS) state that all public sector bodies must test their sites and services regularly. They also require us to publish statements for the sites. These must be reviewed annually. 

It is not specified how often you should audit your sites and services. However, the work that is required for audits can be quite lengthy, depending on the size and nature of your digital estate. As your statements must be reviewed annually, you may also want to audit annually. However, if fixes are due on a site or service, you should have an agreed timescale for the fix. In these cases you should test the fixes when they are applied, and update the statement accordingly.

GDS states that accessibility statements must be reviewed at least once a year, even if there have not been significant changes to the website. You will need to read through your statements, check any outstanding issues, and update the date that it was last reviewed (this is a feature that is required in your statement). However, if you are also auditing annually, you may have other changes that you need to make to your statement as well.

Please note that other governing bodies around the world may vary; they may stipulate a specific frequency of audits. You should always check with your nation's legislation.