All it takes is one dirty misfiring spark plug to decrease your mpg by seven percent. Make that two misfiring plugs and you're down 20 percent in fuel efficiency. With this in mind it is important to change your spark plugs every 30,000 miles or so.
Some spark plugs like platinum and iridium will tell you that you can wait for up to 100,000, but when you are talking about losing that much efficiency over one bad plug, most experts don't recommend going too much beyond 45,000.
Spark plugs are actually very easy to change with just a few tools and an hour or so of your time. Don't worry too much about all the specialty plugs available out there unless you are in the mood for some experimentation. What the manufacturer originally used, is usually what will offer the best economy... they are the ones who tested the vehicle for its optimum performance after all.