How long do semi permanent eyelash extensions last the truth is that a natural eyelash has only a 2 3 month lifecycle in order to apply a synthetic eyelash to a natural hair the natural hair has to be 13 the length of the synthetic hair we are applying which means this hair could be a couple weeks or more into the natural lifecycle with this being said we apply a full set starting at 50 lashes per eye how often will i need lash refills my clients come in for their refills anywhere