If you have the factory battery and live in a cold climate now would be a good time. If you live in a place like Florida you should have another year. BTW if you have a Advanced Auto Parts Store near you, Google Advanced Auto Parts coupons. They have a coupon for 20% off batterys every now and again.
No set number of miles, time is more the issue. A battery these days will normal last 4-6 years.
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