Although the recommended interval is somewhere in the area of 100,000 miles they should be changed about every 50 or 60 thousand. The reason is not necessarily because they lose their ability to ignite the mixture, but because after a certain number of heat cycles, they can sieze into the aluminum heads.

To prevent this, change them at 50 or 60k, change them ONLY when the engine is dead cold, use a VERY thin coat of copper based anti sieze compound on the threads being sure not to get any compound on the end of the plug and torque them to about 13 ft lbs.

Good luck,