Although some of the modern spark plugs may be used for as long as 100,000 miles of driving, you should never leave them unchecked for such extended periods of time. There are different schedules for their replacement depending on the type of engine and the design of the plugs. Usually, they should be looked upon about every 30,000 miles, but it may vary widely depending on the model of your vehicle.
The one thing that is certain is that you should not wait until you notice any signs of the spark plugs being worn off, such as loss of performance of the engine. When you reach that point, it may turn out that the spark plugs will be very difficult to unscrew or other elements of your car may be damaged.