Some people end up changing it as low as 50,000 miles, but this is very rare. 100,000 miles is a good span with the older cars but your car, it should last much longer. A lot of people are finding minimal carbon buildup on the new cars, especially when the ultra low sulfur fuel is used....and good oil too.
Update: I took off the pipe and there was definitely a thin film of black goo on the inside. It wasn't nearly as bad as some of the pictures that I've seen so I will keep an eye on it and see if it gets any worse.
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