Powder portables (i.e. under 20 kilo charged weight) have for decades required discharge testing & new agent and if not extended serviced to BS5306-8 would have to be marked 'not maintained' firmly putting the liability on yourselves.
Wheeled extinguishers to BS EN1866 have no set service regime other than 'manufacturers instructions' and you have more flexibility.
The powder manufacturers only give the product an open ended shelf life (at least 5 years) in dry storage in it's original undamaged packaging as once opened even under perfect filling conditions the powder will have been contaminated, notably by moisture and will be liable to chemical change, environmental effects and caking. Degradation risks will be greater in cartridge extinguishers particularly those without a anti moisture bursting disc.
The testing by discharge is an ideal training opportunity and with a degree of skill required to correctly apply powder on larger industrial risks, including airfield use, then I don't see and issue other than the fact they are all due at once - I'd implement a 20% a year cycle for test discharging.
I appreciate Monnex is very expensive (albeit very, very good having used it at the FSC) -perhaps you should consider Purple K which almost parallels Monnex having a projected 233B rating from a 3 kilo fill, you loose a small bit of the speed by not having the decrepitation of the powder that the Urea complex of the monnex compound gives, but it is still very good, far exceeds BC30, BC Plus or ABC plus and is the agent of choice in the US and US influenced territories where Monnex never really caught on.
