Life of the car can mean any where from like 40,000 to 150,000 miles. Timing chains should be replaced in any rebuild. They do not last forever and sometimes using the term life of the engine confuses people because it makes them think they never need to be replaced. I have replaced the timing chain in my 2001 Dodge Ram 1500 twice. Once when I first bought it because at 100k it was already on it's way out and then again at 150,000 miles when I upgraded it to a double roller. However the chain already had significant wear and slack after 50,000 miles. Same goes with my 95 5.7L Tahoe. I've replaced the timing chain in that as well. Once at 100k, and again at 200k. Both times, the chains had enough wear and slack to warrant having them changed.
At the end of the day, it all comes down to how the engine is designed. If the timing chain architecture is sound, then you'll probably never have to replace it unless something catastrophic happens. However on engines that are poorly designed, chain wear is a more common thing and replacing a timing chain becomes a regular maintenance item just like a timing belt.