I've got two 8D's. They were new in the Spring of 2008. The "House" battery went bad in October, replaced it. The "Start" battery was still good and used a couple of times after replacing the house battery, but it wouldn't turn the motor over in the yard to winterize the first week of November! New one coming in April. Five years seems to be about the normal max for them.
I've always used my batteries until they show signs of failing - BUT, I never rely on just one. The best I ever saw was in my old man's 1960 Chevy - original battery lasted 11 years!
