I suppose if you think about it, a Fire Risk Assessment is a live document - one day you have your fire risk assessment carried out, the significant findings are all documented, then any defects(potential fire hazards) identified can form part of your fire safety action plan for eliminating or reducing the identified fire hazard/s to a tolerable risk.
Therefore from time to time and there is no laid down time stipulated in the current fire precautions(workplace)regulations, a periodic review should be carried out of your FRA, it would be wise to review your assessment at least annually.
As the soon to be withdrawn employers guide states "sooner or later you may introduce changes in your workplace which may have an effect on your fire risks and precautions, eg changes to the work processes, furniture,plant,machinery,substances,building,or the numbers of people likely to be present in the workplace.
Any of these could lead to new hazards or increased risk, so if there is any significant change, you will need to review your assessment in light of the new hazard or risk. and check to see if the existing arrangements are still deemed adequate or if changes are needed.
The periodic review can also help identify if matters highlighted in the original fire safety action plan have been attended to or been overlooked, this is why it is always advisable to have proposed timescales for implementing any necessary improvements as identified by the original or last FRA.