How often do right-to-work checks need to be repeated?

Right-to-work re-checks are only required for employees with a time-limited permission to work in the UK -- that is, anyone whose right to work is tied to a visa, a biometric residence permit, or other document that has an expiry date. British and Irish citizens with an unrestricted right to work never need a follow-up check (the initial check provides a permanent statutory excuse).

For time-limited workers, the follow-up check must be carried out before the current permission expires. The timing of the next check is therefore driven by the expiry date on the permission document -- you check again when the document expires and is renewed. In practice this means the re-check date is the date the current permission expires, not a fixed annual or biannual date. Each employee's re-check date is different, and it moves every time their permission is renewed. This is why a static calendar reminder doesn't work: you need the actual permission expiry date for each employee, tracked and reminded individually.