Let's deal with the why first. When your spark plugs are old then your engine doesn't run as well, it burns more fuel and it's harder on the engine.
Now the when, that depends on your engine. They are making the parts that go into engines better all the time, so it really depends on the manufacturer. To keep costs down for them many manufacturers will have you change your spark plugs every 30,000 miles, others need to be changed every 60,000 to 90,000 miles.
When we see your vehicle we will make it a customized maintenance schedule so you know when your services are due. Give us a call and let us take care of you.