There will be a wear mark in the middle of the tread. The thickness of the wear mark is 2mm. If the thickness is worn to 4mm from the limit mark, which is 6mm from the deepest part of the groove, the tire should be replaced.
Generally, the service life of tires is 3-5 years, but it is not recommended to change tires based on time, because everyone's car habits are different. If you look at the mileage alone, 50,000 kilometers is more appropriate.