How often should I replace the spark plugs on my Toyota 4Runner?

I have a 1997 4Runner that has 30,000 Miles. I don't believe I need to replace the plugs till at least 60,000 but the dealership says that I should. I believe that this is just them trying to get more money from me. How often should I replace the spark plugs on my Toyota 4Runner?If ya get it serviced every 6mths prob around then or with oil changes