In November, when I put mine in for its first MOT since I bought it, it failed on all four tyres. They had loads of tread depth left, close to half their original level, but they were all out of date, and one was badly perished on the sidewall. I didn't ask how long out they were, or when they were made, but even though my car is fairly low mileage for its year, I think that's a strong indication that a set will cover a lot of miles.