E-cigarette coils are the part of the tank that turns your e-liquid into the vapour that you inhale. Generally coils can last around 2-4 weeks depending on how you use your e-cigarette. You will know your coil needs changing when you take a draw on your e-cigarette and can taste burning.
When changing your coil for a new one you must always ensure that it is primed. This can be done by either putting a few drops of your e-liquid on the inside cotton/mesh of the coil. If you find this too fiddly then simply fill your tank with your e-liquid and let it stand for at least 10 minutes to soak up some of the liquid.