Again, depends on things such as the location etc. Domestic properties are normally tested every ten years. Or when there is some work done which require a certificate. Or when you are concerned about a faulty installation
It involves testing every circuit at the fuseboard with an installation tester. It will involve things like removing lamps and dimmer switches etc, but is generally unobtrusive. The tester will measure loop impedance, insulation resistance and continuity, among other things. The real work comes when a fault is found!