merlin said:Chains are life long, but the chain will have a tensioner - that'll need replacing some time between 100k and 150k. Handbook will tell you more.
They are not always life long and can stretch with time. My dad had a chain snap on his w123 series merc, although it did start to get louder before it snapped. Its a mith they are life long, they just last a lot longer than belts. They should be measured for stretch then replaced if out of tolerance, just a lot of cars never make it to the point of the chain needing replacement(although a lot more do now as cars last longer).