From the linked document above: "You need to keep a record of each illegal content risk assessment you carry out", "service providers may fully review their risk assessment (for example, as a matter of course every year)"
And links to a guidance document on reviewing the risk assessment[1] which says: "As a minimum, we consider that service providers should undertake a compliance review once a year".
[1] https://www.ofcom.org.uk/siteassets/resources/documents/onli...