How often you need to PAT Test is really determined by your appliances, users and environment. If someone tell you that once a year should be fine. It may not be!
When you start PAT Testing, you need to look at each appliance and determine the risk. The appliance alone does not determine the risk, but it is a factor. So for example if an portable electrical appliance is used with water, it is automatically high risk, but if it is plugged and unplugged very rarely, stored carefully and in good condition, the risk is reduced. Where an appliance is used with water, unplugged and plugged frequently and dragged around by it’s lead every night by an exuberant cleaner, a more frequent inspection program would be appropriate. Every day items like cleaners and kettles can really take a bashing and as such need inspecting more frequently.
Where you have static desktop computers you can probably include these in your inspection regime far less frequently, providing they meet the other criteria for low risk environments and users.
More information can be found in the HSE document INDG236