Spark plugs

I use the NGK Iridium plugs, they work great, I changed them at 7500 miles and I know have 13300. I guess its time for a change. They provide a very hot spark.
is it bad not to change your plugs at every 7500? i am over 14000 on the original ones...why don't they last as long as cars do? plus i haven't had any problems on the stock ones....

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