That varies considerably. The atomizer is the most delicate component of an e-cig. On average, the Joye* 510 atomizer is more reliable than many others (although, during the past couple months, I've read of problems). Mine have been lasting me for months.
* All my comments apply to Joye rather than clones.
Make sure that you have at least one backup (in addition to the two, which came with your starter kit).