Serpentine Belt Replacement
 

I am curious how often should I replace the serpentine belts....Is it based on mileage or time interval or if there are some visual signs like cracks to indicate it is time for replacement.

I read somewhere that it would last 100k miles in general.

check for cracks and life time, I don't known. Mine went when I was tracking last year, no real visible signs. When I told my tech he looked at me funny. So in my case, it probably was the tracking the lead to failure. I have about 75K on my Z.

I imagine it would show some cracks before it actually break under normal driving condition?

I am curious if it caused any engine damage when it broke on the track?

My friend lost the motor on his BMW when the broken belt wrapped around the crank pulley and seized the motor.

Nothing to the engine, I did lose the power steering unit. It was quick and I had plenty of time to pull off. I was more concern about the heat coming off the brakes, then the engine. But everything was ok, once the car got back together. The belt literal disintegrate, found bits of it every place.

The info. you're looking for is in both the owners manual and workshop manual.