The recommended interval for changing spark plugs on a Harley-Davidson can vary depending on the model and specific engine. However, a general guideline is to replace the spark plugs every 10,000 to 15,000 miles (16,000 to 24,000 kilometers) or approximately every two to three years. It's important to consult the owner's manual or the manufacturer's recommendations for your particular Harley-Davidson model, as they may provide specific instructions or variations based on the engine type and other factors. Additionally, if you notice any signs of poor performance, such as misfires, rough idling, or difficulty starting, it's a good idea to check and replace the spark plugs if necessary, regardless of the recommended interval.