This varies from manufacturer to manufacturer. Harley recommends changing your motorcycle spark plugs every 10,000 miles. This is not because the spark plugs have gone bad, it is simply because they are not as efficient as they were when they were new. Spark plugs wear over time. The edges of the electrodes will eventually burn down. Spending a few dollars to keep your motorcycle running like it should is a good idea. Check your manufacturer recommendations to determine when is best to change your spark plugs.