The main factor when determining how often electrical items should be test and tagged is the environment they appliance is located and used in. We recommended the following as a minimum requirement which is roughly what is required by the AS/NZ3760:2010 standard requirements.
3 months: building, construction and demolition
6 months: factories, warehouses and production
12 months: an environment where the equipment/supply cord is prone to flexing or open to abuse
5 yearly: an environment where the equipment/supply cord is not prone to flexing or open to abuse