How to define overhaul period of transformer?

A major overhaul of the main transformers of the power plants and substations operating under normal conditions (i.e. operated according to the nameplate data of the manufacturer's rules and without any abnormal phenomena) shall be carried out within the fifth year of commissioning, in the future, it will be repaired once every 5 ~ 10 years. The detailed overhaul should be determined according to the operation condition and the test result. Other transformers in power plants and substations.