Depends on the batteries and how hard you use them. I have been using 650 ma evod batteries for 9 months with no noticeable decrease in capacity.How long shod you use the same set of batteries till thy don't hold a charge our 2-3 mos?
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On 18650's if you use them at lower wattages usually about 300 charge/discharge cycles is the norm. if you use them hard you can probably half that.
If you nocice ANY leakage or swelling ie they are hard to insert in the mod. Stop using them immediately.
The wrapper must also be kept intact if torn rewrap or discard.
There are vids on youtube about rewrapping them.