http://auto.howstuffworks.com/under-the-hood/car-part-longevity/how-long-ignitions-last.htm
Short answer: A lot longer than they used to. I learned a ton about the differences in engine between the 1970s, or “the bad old days,” as one of my interview subjects called it, and the ignition systems in place today. Read all about it on HowStuffWorks.com, in an article published in October 2009.