Each AS1841 standard fire extinguisher must be serviced every three years in accordance with
AS1851 for a fire extinguisher in an adverse operating environment. If the extinguisher is compliant
with another acceptable standard, it must be serviced every two years. This servicing must be
undertaken in accordance with the procedures laid out by the manufacturer and/or the relevant
standard (e.g. AS1851 for AS1841 extinguishers) and will include a hydrostatic pressure test of the
extinguisher body. A report from the servicing agent must be supplied to a scrutineer on demand as
proof of a service being completed. A service tag will not be accepted as proof of the extinguisher
having been serviced

Thus most of the extinguishers used it means 3 years from the date unless tested.  The testing costs for small extinguishers is usually more than the cost of replacement.