There are 3 parts of the e-cig that need to be cleaned and maintained - the battery, the atomizer, and the charger.
These parts should be cleaned periodically depending on how often you use the e-cig. Some people say that a once a week cleaning is good but you must ensure that you have enough supplies before cleaning, as you will not be able to use your items until they are dry. This might take a full 24 hours to be sure. A good rule of thumb is to stagger your cleaning. For example, one week you clean A, B and C on Monday. Then on Wednesday you can begin using A, B and C, and clean D, E and F.
The type of E-Liquid can severely alter your cleaning schedule. The more viscous your liquid is (how thick) the more gunk will build up. Also the darker the liquid seemingly makes the wicks clog much faster and need much more frequent cleaning. Some people use only certain cartos/clearos/etc. for certain liquids. For example, an atomizing unit you know is super easy to clean or does not have wicks you might use for your thick, dark liquids. Many people will find each atomizing unit will make your liquids taste certain ways. For example, you might like your fruity flavors in one that burns cooler, and your coffees in one that burns hotter. Never expect the same flavor from different items, and they may also differ from battery to battery!