Just replaced the battery in our 2001 Grand Cherokee. So, the orig battery was 2001, then replaced at 2006, now again at 2012.
This would imply a general life of about 5 or 6 years.
Since I was stranded in a Home Depot parking lot, and had a really hard time jumping the battery and then keeping the engine from stalling out... for some reason...
Would it be better to be pro-active and just replace batteries every 5 years, or wait for the eventual internal failure and being stranded... even with always having jumper cables :)