Portable fire extinguishers are intended as a first line of defense to cope with fires of limited size.  Annual inspections are designed to ensure that the extinguishers will operate as needed in the event of a fire emergency.

The National Fire Protection Assosicate states the following for Commercial Properties/Situations:

NFPA 10 7.3.1.1.1 ~  Fire Extinguishers shall be subjected to maintenance at intervals of not more than 1 year, at the time of hydrostatic test, or when specifically indicated by an inspection or electronic notification

NFPA 10 7.2 ~  Fire Extinguishers shall be inspected when initially placed in service and therafter at approximately 30 day intervals.

Manufacturers specifications indicate that every 6 and 12 years from the date of manufacture, extinguishers need to be taken apart, inspected and tested.