This is because the rubber in the timing belt degrades over time, regardless of how many miles the car has driven. A belt that sits unused for 10 years is more likely to fail than one that's driven regularly but is less than 10 years old.
Therefore, while there's no mileage recommendation, check your owner's manual for the specific time-based interval (likely 10 years) for your vehicle. It's best to err on the side of caution and replace it at or before the recommended time interval. A broken timing belt can cause catastrophic engine damage.
