When do you need to Replace Tires?
Tires are typically replaced when their natural lifespan has come to an end. There are however any number of factors that can affect this including storage, temperature, surfaces they are driven on, how aggressive of a driver you are, punctures, etc.
The usual recommendation for replacing tires is when they no longer feel safe to the driver. You can see when they are legally worn out using the tread wear indicator. A typical set of tires under normal driving conditions will last approximately 40,000 miles or 64,000 km. Some tires of-course are designed for much longer, even up to 80,000 miles or 128,000 kilometers.