As with the hair on your head, eyelashes follow a natural cycle of shedding and regrowth. All of your lashes are at a different stage at any point in time, which means that some of your lashes are getting ready to fall out while others are experiencing a period growth (this is true if you have eyelash extensions or not). Since an individual extension is applied to an individual lash, that extension will fall out on its own when the eyelash sheds from the lash line. If you aren’t exacerbating this shedding process by rubbing your eyes, applying mascara, or compromising the glue with oily eye products, your lash extensions should last up to a month before you need a “refill” appointment. 

Unless you’re transitioning off of extensions, refills are necessary every 3-6 weeks to maintain the best result. Since each of your individual lashes is at a different stage in the growth cycle, you’ll start to see a more scattered appearance after 3-6 weeks as new natural lashes come in and lashes that had an extension attached begin to fall out.