A timing chain is something that you don't wait until it breaks to change it out, unless you want to rebuild the whole motor.

I changed mine at only 70K because I happened to be doing the waterpump and it was accessible.

Mine had the cheap pot-metal (or aluminum?) cam sprocket with nylon teeth. Some of the teeth were cracked and a few more broke off when I dropped it on the ground. I put a steel gear in there.