My suggestions. To save money, drive until the ...

tires are not safe any more, 4 years or 35,000 to 70,000 miles depending on the tires. To get a comfortable and safe ride, changes tires every two years or 2/3 of the design mileages. The tire rubber gets harder and brittle over time under the sunlight and O3 in the air. After 4 years, usually they are not safe any more. But differnt tires may vary a lot. Just want to give you a general idea.