Your new eyelash extensions should last approximately 2-4 weeks as long as you care for them properly. This would include avoiding any sort of oily eye make up remover and not sleeping on your face. Do not believe the false promises of those who claim that they apply permanent eyelash extensions. This procedure is semi-permanent.
The life cycle of a natural lash is just under a month, about 25-28 days. Your synthetic lashes are attached to your natural lashes, so as your lashes go through the entire growth cycle and fall out, your extended lashes will fall out with your natural lashes.
You will know how fast your lashes will grow based on how fast your nails and hair grow. In addition, what you do for work as well as the rest of your daily habits can play a part in how long your lashes will last. Certain industries where you are exposed to the elements or in a work atmosphere where heat or steam can take a toll on your synthetic extended lashes. This is a huge contrast, especially compared to someone who works in a temperature controlled office complex. Do not believe the false promises made by shops that claim that synthetic eyelash extensions will last two, even three months. Only people who lack experience will make these sorts of claims.