Replacing ignition coil
 

Does the performance of a coil deteriorate over time, or does it work perfectly and then just fail?
What is the typical useful lifespan of a coil?
Is there any benefit to replacing a coil as a maintenance item at some interval, or should one just wait for it to stop working?

They typically have very long lifetimes. They can fail from overheating, bad contacts causing overheating or an ignition module sending an incorrect signal.

You should not have to replace the coil unless it fails. they are typically not replaced as a maintenence item. Besides, the new replacements have been shown to fail more often than the originals.