When do portable extinguishers need to be hydro statically tested?

it Depends about the type of cylinder and the gas inside it. for Example and based on NFPA 10 ,Table 8.3.1 Hydrostatic Test Intervals for Extinguishers:

- the Carbon dioxide CO2 extinguisher tested every 5 years

- the  Dry powder extinguisher tested every 12 Years. 

periodically : 5 years for CO2 tube and 12 years for dry/wet powders or if you see any damage to the tube after and event (dropped, heat/fire/ strucked etc)

In order to check whether cylinder can take the stored pressure and serve its intended use. Also to check for any physical damage, corrosion, leakage and clogged nozzle. To avoid any mishap.

Fire extinguisher cylinders are tested by the manufacturer. Testing is carried out after fabrication. All cylinders go through this process. Hydro or pneumatic test can be carried out. The quality control will dictate what medium to use based on the requirement.

Testing as I have mentioned is a requirement therefore it should be done otherwise the product will not make it to the market.