Our natural lashes have a regular growth and shed cycle therefore the extension will fall out with the natural cycle, which is why they are called semi-permanent. Everyone's lash cycle is different and each individual lash will be at a different stage of the cycle at different times. 
They do not damage the natural eyelash which can be the case with traditional self-applied false eyelashes. They will naturally darken your lashes as they will be fuller and more visible. 

When cared for and with regular maintenance, lashes can last up to 12 weeks. To maintain the fullness of the lashes and to keep their perfect appearance, it is recommended that they are retouched every two to three weeks.