Updated: Jan 2, 2020
As long as you keep paying for them! I'm kidding! This is by far the trickiest question to answer because there are literally 100s of factors that can affect someone's retention that neither of us have control over (ummm hi? Did you know humans go thru a major seasonal hair shedding twice a year? Yeah, we do!).
So here's the lowdown on how long eyelash extensions last. The adhesive has a "cling life" if you will of about 6-8 weeks. Meaning the bonds of the adhesive will start to break down around that time frame where the glue becomes brittle and worn down. BUT our eyelashes have a growth cycle just like every other hair on our body and will shed out naturally (extension or not). SO you can expect to lose anywhere from 3-5 lashes a day if you're healthy and have no underlying health issues.
So to keep your lashes looking full, I recommend coming in every 2-3 weeks to replace the lashes and/or extensions that have shed out and to put fresh new ones on. It's very similar to the frequency of acrylic nail fills. Your natural lashes grow, shed and grow a new one and that is happening constantly across your lash line.