From what I understand, they don't have a known lifetime in either miles or time. Failures seem to be a random event, and common practice is to replace just the bad one. I had my first coil problem at about 165k, and just replaced the bad one.

 

Ya at 30 ea it isnt a killer to replace them all. And i have even seen an entire Bosch set on Ebay brand new for $190. So that is well worth it. I will keep them as spares in case another car comes in with a failed coil.