Time is a pretty meaningless concept when applied to atomisers and cartos - what actually reflects true "life" is the amount of juice put through one before it wimps or dies.   A good atty should take around 40ml of juice at least before needing replacement.  (I know someone who vapes 10ml a day - so that's a "life" of 4 days!   I vape 2-3ml per day so life is between two to three weeks or more...)