A clearomizer/tank with a non-replaceable coil head can last from anywhere between one and four weeks (depending on frequency of use and type of e-liquid used). After this period, the coil within it will generally burn out and will need replacing. Now days most clearomizers/tanks will have removable coils (to save you needing to buy a whole new clearomizer/tank), and coil heads typically need replacing every 2-3 weeks depending on how often you use your e-cigarette. You should replace the coil head when you notice a slightly burnt taste, or when the e-liquid you're used to tastes different.
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