The honest answer is that nothing in fire safety law names an interval, and anyone who quotes one without saying where it comes from is quoting a British Standard they may not have read. What the law asks for is a system kept in efficient working order under a regime that suits the building. The regimes people actually follow come from two standards, one for workplaces and one for homes, and they differ because the risks differ.
The workplace answer
For a non-domestic system designed to BS 5839-1, the recommended pattern is a brief user test every week using a different manual call point each time, plus periodic inspection and servicing by a competent person spread across the year. The point of rotating the call point is that a system tested from the same device for five years has proved one device five hundred times and the rest never. Your risk assessment is where any departure from that pattern has to be justified.
The home and rented home answer
For dwellings, BS 5839-6 is the relevant standard and the expectation is much simpler: the occupier tests each alarm with its own test button, typically weekly or monthly depending on the grade of system, and the alarms are replaced at the end of their stated service life rather than when they fail. In rented homes the landlord has separate statutory duties to ensure alarms are working at the start of each new tenancy, which sits alongside the standard rather than replacing it.
Why nobody can give you one number
A hotel with sleeping guests, a school, a warehouse with an aspirating system and a two bedroom flat are all covered by the same duty and none of them should be on the same schedule. That is why the Fire Safety Order describes the outcome, efficient working order, and leaves the method to the assessment. If an interval is being imposed on you, ask whether it comes from the standard, from your insurer, or from the service contract you signed, because those three are frequently different.
What to ask your contractor
Ask which standard and which category the system was designed to, what the service visit covers and how many visits a year that implies, and whether the weekly user test is yours to do or theirs. Ask what happens to the certificate if a fault is found and not fixed on the day. A contractor who answers all four without reaching for a price list is describing a maintenance regime rather than selling a visit.
Questions people ask about how often should fire alarms be tested
How often do fire alarms need to be tested by law?
The law does not set a frequency. It requires the responsible person to keep the system in efficient working order and in good repair. The frequencies in common use are recommendations from BS 5839, adopted through the fire risk assessment.
How often should you check your fire alarm at home?
Test each alarm with its own button on a regular schedule you will actually keep, and read the manufacturer's stated service life. Most domestic alarms are designed to be replaced after a set number of years whether or not they still sound.
Does the service visit replace the weekly test?
No. They prove different things. The service visit checks the whole installation including the parts a user cannot reach; the weekly test proves that the alarm still raises the building today, which is the thing that matters between visits.