Apple says that iPod batteries will maintain up to 80% of their capacity through 400 charge/discharge cycles. A charge/discharge cycle is just what it sounds like - using the iPod until its battery is substantially run down and then recharging it. How long that translates to depends on how often you need to full recharge your iPod. Light users could get three or four years of use out of a battery, while heavy users might only get 18 months.
Apple recommends that you perform at least one charge/discharge cycle a month, as iPod batteries benefit from this and retain their ability hold juice longer that way.
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