When to replace spark plugs?

This question totally depends on what kind of spark plugs you have in there in the first place. I personally have high end spark plugs which don't need to be changed for 75,000 miles. Unless you see signs of failing spark plugs, they don't need to be changed. Having a general knowledge of what these may be will be needed to know. For instance misfires are often caused by failing spark plugs and or wires. If your engine is running smoothly and gas mileage is good, don't concern yourself. Shops and technicians will lie to you and tell you otherwise. They have kids to feed, who can blame them.