The eyelash extensions will last for a few weeks, or anything up to 5 weeks, depending on the length of your natural lash cycle and your attention to aftercare. You can read more about this here.
Other things that affect retention are oilyness in the skin and lashes, which breaks down the adhesive bond and the lash shedding cycle, which happens with all of us several times over the year, just like the hair on your head. Many of my clients have ongoing extensions and on average, they come back at week 3 to have an infill session and at this point most would have half, or more of their extensions intact. Some last longer and others maybe come at 2 weeks because of a shorter lash cycle, or because they want their lashes to be as full as possible at all times.