A BITOG old-timer chiming in here after many years, on many vehicles.
As to changing out the long-life (100,000 mile) plugs on many modern engines, it is usually a good idea to not let them go that long, for two reasons. Besides the increasing risk of thread seizure going out up to a decade in service on some examples with aluminum heads, spark plug performance and efficiency does not sharply plateau over the service life. It is a gradual decline, and while visible plug erosion and fouling won't be noticed until closer to the end of the service life, the declining spark performance will begin to stress the emission control systems on many cars on the back end of that service curve. I don't want to get into the technicals and engineering white papers, but a 70,000 mile plug does not fire like a new one.
So yes, the plugs in these 'long-life' situations will provide 100,000 miles of satisfactory service. But the wear they will cause to other systems, not to mention the risk of galling and seizure in aluminum heads at longer time frames, counsels to do it sooner. I've found a happy medium at 60,000 mile changes or 8 years maximum for those types of plug installations. I do not recommend pulling non-tapered base plugs (most types today) for repeated inspections; it is hard to maintain a gasket seal on subsequent installs, and higher torques become necessary. It is also a lot of wasted labor. New plugs are cheaper than labor.
As far as using anti-seize on plugs, there are three factors to consider: the head material, the length of time the plugs are expected to remain in service, and observing torque best practices. As many modern IC engines now use aluminum heads, the risk of seizure in long-term installations is very real. Many manufacturers now provide both time and mileage recommendations for that reason. So for an engine that is not going to be driven normal mileage, the decision is either to replace the plugs on time (before they have adequately worn), or to replace the plugs at longer intervals using an anti-seize product.
Because the risk of seizure is greatest with the same head materials that are also prone to stripping on over-torquing whether lubricated or dry, a torque wrench is probably good practice in any event. And because a torque wrench is being used to start with, it is easy enough to observe the usual industry standards for torque reduction when using lubricated vs. dry threads. And that is a 25-30% reduction in the torque specification. As long as the compound is not over-applied, it will not cause problems. Just a tiny dab a couple threads up from the electrode floor. And concerns about the compound in a combustion environment are unwarranted. The minimum specification for standard aluminum-based products is 1,600 to 1,700 F., far higher than anything the spark plug will ever encounter in service. If those temperatures are unsatisfactory, then moving to a nickel based product is the answer.
So for a low-mileage aluminum head engine that may not see a plug change for 8 or 10 years, we use a very small dab of standard aluminum AS, and de-rate the torque by 25%. The plugs always operate fine, and they always come out without issue, even after a decade.
Bear in mind that the use of AS is a very controversial subject, and the plug manufacturers recommend against it primarily for the other risk, over-torquing. But anyone who has encountered a seized plug will see the value of AS going forward, but with a correct torque de-rating.
The only exception to this is with the newest DI gasoline engines, where proper plug indexing can be critical to engine operation. In these situations, the best practice I have found is to stick with Genuine manufacturer replacement plugs, which have a higher quality assurance for correct final index position than the OE equivalent lines do, and to precisely torque the plugs to specification. The latter practice is extremely difficult to pull off where AS compounds are used. So for these latest engine designs, we simply torque them in dry, and make the change on time or mileage as directed.
We thought about none of this 50+ years ago. Back then, the plugs were taper-seated into cast iron heads, and hand torquing by feel was the standard of the day. Of course, back then, we were changing them every 12 mos./12,000 miles, along with points, condensers, and rotors. Ah ... the days of dwell meters and timing guns.
ngksparkplugs.com
This right here. This should be listed in your maintenance manual. Regardless of how long they might last there is usually a specific interval required for the manufacturer emissions warranty.