"The responsible person must review the assessment made under paragraph (1) regularly to keep it up to date and, in particular, where there is reason to suspect that it is no longer valid or there has been a significant change in the matters to which it relates."
Regulatory Reform (Fire Safety) Order 2005, Article 9(3). The word "regularly" is not defined in the legislation. In practice, enforcement officers expect annual reviews as a minimum for most premises.
When a review is definitely required
Regardless of when the last review was, a fire risk assessment must be reviewed immediately if any of the following occur: the premises are extended or altered, occupancy increases significantly, new processes or activities are introduced, new hazards are identified, a fire or near-miss occurs, the responsible person changes, or any enforcement action is taken or threatened.
What "no longer valid" means in practice
An enforcement officer visiting your premises can decide that your assessment is no longer valid if the current conditions differ materially from those described in the assessment. This includes changes as simple as new furniture arrangements, new equipment, or changes to the number of people regularly on the premises.