Your tyres should be replaced when the tread depth of each tyre has worn down to a minimum level. This varies depending on the type of vehicle, but is generally between 1.6 and 2mm. If you have any doubts about the condition of your tyres, it's important to get them checked by a professional and replaced as soon as possible if needed. In addition to regularly checking your tyres, you should also make sure they are properly inflated according to the manufacturer's recommendations. This can improve fuel efficiency and provide a smoother ride, as well as ensuring better handling and safety on the road.