We suggest following
the manufacturers recommendations
 

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Follow the car makers recommended service interval.
 

Engine Tune

The nature of an engine “tune-up” has changed considerably over the years. No ignition points to adjust or change. No carburetor to adjust.
We still change the spark plugs, but not every 5,00 to 12,000 miles as of old. It was common when I started working on cars for 'minor tune ups' be done and plugs cleaned at 3000 - 5000 miles.
Some modern spark plugs now last up to 100,000 miles.
A 4-cylinder 2009 Hyundai Accent has its first recommended spark plug change at 30,000 miles for both normal and severe conditions. Likewise, a 4-cylinder 1990 Honda Accord recommends a plug change every 30,000 miles or two years.  
Several manufacturers advertise "long life" spark plugs. Using precious metals such as silver, platinum, gold and iridium, these plugs have an advertised life close to 100,000 miles. An 8-cylinder 2010 Hyundai Genesis has a recommended plug change interval of 97,500 miles, driving in normal conditions, using iridium-coated plugs.
Today the best advice is to follow the car makers recommended service interval.