Some people recommend changing the copper spark plugs every 30,000 to 50,000 miles which is not gonna hurt anything and will just help increase the performance of your engine.
But I only change my spark plugs every 100,000 miles which is around every 5 to 6 years depending on how soon I put those miles on the vehicles.
If it's been 3 years or so and you haven't changed your spark plugs then it's probably time to change the spark plugs just as a tune up.
A good set of spark plugs though should last 60,000 to 100,000 miles and if you want to just maintain your vehicle and change them as preventative maintenance then you can do so as soon as you want too.