Well I have always "tuned up" my cars that had standard plugs every 15,000 miles. I changed my two cars that had platinum plugs (claimed 100,000 service interval engines) at 50,000. In both of the later cases the plugs still looked very good.
There are some cars/engine combos that are plug type sensitive. When I had my 86 Mustang SVO I ran only Motorcraft original spec plugs and ignition components. It is a widely known fact among SVO enthusiast that most ignition misses and pinging under boost isses could be solved by giving a fresh tuneup with Motorcraft ingntion items and not running gimmick or specialty ingintion components. Especially when running higher boost pressures that most of us SVO owners run/ran. Those cars were HARD on ignition componets (Fords problem prone TFI module not with standing)
And of course my van just got new plugs and wires courtesy of the dodge warranty with 10,000 miles on them.
2004 Inferno Red, Anniversary Edition, Dodge Grand Caravan SXT, 3.8 V6
2002 Mercury Sable LS, 3.0 24V Duratec V6
Previously owned vehicles
99 4WD Ford Explorer XLT, 4.0 SOHC V6
86 Ford Mustang SVO, 2.3 FI, TC, IC, I4
95 Ford Contour SE, 2.5 24V Duratec V6
94 4WD Ford Ranger XLT , 4.0 V6
89 Pontiac Grand Prix SE, 3.8 V6
91 4WD Ford Ranger XLT, 4.0 V6
81 Chevy Caprice Classic, 305 V8
88 Ford Mustang 5.0 V8
79 Ford Mustang Cobra 2.3 TC, I4
75 Ford Elite 351W V8