My name is XXXXX XXXXX I will answer your question. In the old days when we were driving 289 and 302 V-8's Ford and most manufactures were using camshaft gears that were steel in the center and nylon or plastic where the chain rode on it. This was a attempt to keep engine noise down and eventually the upper gear would wear out and the chain would jump. On the old engines it seemed like about 100,000 miles was the magic number depending on driving habbits and service intervals.
Todays engines use steel gears, hydrallic chain tensioners and nylon guides to keep the chain tensioned and quiet. These systems do not have a service interval. If there is a problem with engine timing, then you service the timing system, otherwise, no service or replacement is nessessary except for performing normal oil changes.