As long as six weeks – as the eyelash extension generally lasts until the natural lash falls out in its natural cycle. However it is advisable to have some infills applied approximately every 2.5 – 3.5 weeks (depending on how well you take care of them) to make sure your eyelashes always look fabulous, as by this stage some would have fallen out as new lashes have grown through. How long your eyelash extensions last also depends on how you care for them (i.e. by removing makeup correctly, not thrashing them with heaps of mascara, and not tugging/pulling at your eyelash extensions). Some clients who take extra good care of their lashes get 4+ weeks before they need infills!