When should I replace the spark plugs?

The book shows that you should replace the spark plugs at 97,500 miles under normal use. However "normal use" is actually not very common if you actually drive the car. Instead most people actually fall into the special or severe usage slot of the service tables. However Hyundai doesn't show a real number for that. So I would probably fall back onto the normal methods and say that for severe usage you should cut the mileage for them in half. So instead of 97,500 I would change them every 48,000 or so.
The nice thing is that they are easy to change on that engine unlike many newer ones.
You need to buy the new plugs then it's a simple, remove the connector from the coil, remove the bolt that secures the coil. Remove the coil. Then blow out the plug access tubes. Then remove the old plug using a spark plug socket and extension. Next verify that the new plug is gapped to 1mm (because these are iridium plugs they shouldn't need to be adjusted but it's possible that they were damaged in the box). Then you install the new plug, tighten it to 10-15 ft lb. Apply a dab of dielectric grease into the coils rubber boot that goes over the plug and place the coil back into place on the plug. Then put the retainer bolt back in the coil and tighten to 8 ft lb and reconnect the electric connector. Repeat that for the other three plugs and you are finished.