I've always considered that a question kind of like, "how long should a man's legs be?"
Answer - long enough to reach from his hips to his feet.. Depending on how the
engine was treated: infrequent/never oil/filter changes, high temperature or very dirty/dusty
environments, lots of revving/drag racing behavior, and how good the chain tensioning
mechanism is can all play into it. I've seen some severely abused engines need a
new timing chain with only 40-50K (miles) on them, others have gone 200K (miles) and
still been within operating spec.
And sometimes, though it is fairly rare, they can just wear prematurely due to
a manufacturing defect or assembly error at the factory relating to the chain
itself, or one of the components it interacts with (timing gears, tensioner block,
cam, etc)
