The approximate lifespan of each refillable tank is around a week onwards. You may find that they last longer as these are approximations of course and are just intended to give a guide.
These are a throwaway item, so when it needs replacing, save any liquid (a blunt syringe is great for this), throw it in the bin and replace with a new one.
General Clearomiser Information
How Will I Know When A Clearomiser or Atomiser Needs Changing?
Clearomisers will deteriorate with use, so a new atomiser head will give a better throat hit than a used one. When they have reached the end of their life, they can often generate a strong burnt taste that will not go away, which is an indication that it's time to change it. They may develop leaks due to the heating/cooling cycle, especially on throwaway (non repairable) types. They can also simply stop working of course.
Life Expectancy
Clearomisers, as with any other atomiser, are a consumable item. They represent one of the on-going costs of using electronic cigarettes and will typically last between a few days to a few weeks of use. This may seem a very large range, but it really does depend upon a number of factors. You should expect at least a few days from an atomizer with average use, but they can last much longer. Some people measure the life expectancy based upon the amount of liquid that goes through them, stating that an atomizer will last for 30 or 70ml of liquid, while others measure by the number of puffs taken.
If an atomizer only lasts a few days or so then it does still represent a huge saving on traditional smoking costs but the problem seems to be that they are quite a solidly engineered unit and people are often surprised when they fail (even after many weeks!). Some types ('rebuildable' types and others) allow the atomiser head (heating element) to be changed when this burns out, often making this type an even better value for money choice.