Dependant on juice, the way you vape and so on, coils just get gunked. E.g. the more dry burns you get the faster your coils will gunk, whilst using certain juices can do it very quickly. Generally 4-8 days is a fair time for a coil to last with an average juice (at least on an EVOD/Protank).
The best and most efficient way to keep your EVOD coils in top form is to wash em, dry it off quickly and touch some tissue to the exposed ends of the wicks to suck up excess water, then dry burn slowly with the airway funnel/cap off till it consistently glows. This process should be started with a low voltage (if using a VV battery) as you may fry your coil. After it is bone dry and between burns give it a sharp blow to remove any dried impurities and you're good to go! Doing this can really extend the life of your coils to weeks and even longer.
Be aware with the EVOD style heads that they come factory standard with a flavour wick (bit of wicking that lays over the coil) and you must take this into consideration when washing. If it is too dirty, becomes misshapen or distorts in the chamber it can give you problems. It is possible to remove this flavour wick entirely (most people recommend it anyway), but it can be a bit hit and miss whether it improves your vape or just makes the atty more temperamental i.e flooding! With coils that have been kicking around forever because of good maintenance, you will probably have to reshape the coil with a tool as it can get a bit bent.
Hope this was helpful and good luck..