As mentioned above, waiting until mileage drops to replace your spark plugs is a bad, bad idea. That is like saying you are going to wait until you hear a rod knock to change your oil. Minor misfires are not always felt by the driver, and in addition to washing down the cylinders, contaminating the oil, and causing excessive carbon build up, the unburned fuel gets dumped into the exhaust system, where the catalytic converter does its job and tries to burn off the excess fuel. If too much fuel ends up being fed to it, it will overheat, the catalyst bed will break down, and now you will be spending $1,500 to replace a plugged converter.
Also, the longer you leave your steel-threaded spark plugs in your aluminum cylinder head, the more likely they are to become seized. As a Ford technician, I love these scenarios because it means $$$ in my bank account. Of course, the neglectful car owner usually doesn't share my point of view.
In my 23 years as a tech, I use these guidelines:
Spark plugs every 60k miles, regardless if they are "100k" plugs or not.
Trans, transfer case, and PS fluid every 60k for light use cars, 30k for trucks or hard car use.
Axle fluid every 50k.
Coolant is good for 100k or 5 years on the first fill typically, then cut that in half for subsequent changes.
Tires rotated every other oil change for freeway driving, every change for city driving.
Air filters vary wildly depending on the vehicle, but I put a limit of 15k on most. They are usually cheap enough to warrant changing at that point.
HVAC filter every 15k. Most people don't even know they have one.
PCV valve every 60k, although many now are fixed orifices that never need to be changed.
There are variations and exceptions, but these will give a good balance between cost and performance for most people.
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