A 110v extension that is used in a harsh environment, such as on a building site, is likely to receive more abuse than a PC in an office and would, therefore, require more frequent testing. Similarly, if the company has developed a strong health and safety awareness which ensures that users report damaged equipment then frequency between inspections may be lengthened. However if that awareness is still developing and equipment is abused, then more frequent testing and inspection would be necessary.
The Institution of Electrical Engineers (IEE) has produced suggested initial frequency guidelines which divide premises into distinct categories and list types of equipment in those premises along with the frequency between inspections and tests.
To create an exhaustive list of every appliance and every environment is not possible and therefore we would encourage you to contact us initially for a no-obligation discussion about the types of equipment that you have and an appropriate schedule for testing and inspection.
As a result of there being no rigid laws as to when equipment has to be inspected and tested we would encourage you to remember that it is the responsibility of the duty holder to ensure that electrical appliances are safe and so the testing frequency guidelines of the IEE are a good indication of what you should be considering.