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What years/miles have you guys had to change at? I have an 03 with 110000 miles and still original but I realize by posting this I may have jinxed myself!
here in the midwest I believe the average battery life is 36-48 months
call your local battery store and ask them.
also keep in mind that its possible one of your batteries is bad allready so your alternator is working overtime to keep the one battery charged and try to charge the dead battery which will burn out the 400-500 dollar alternator in no time.