The age old question!
And unfortunately, it doesn't have one definitive answer...
Firstly, you need an Eyelash Artist who knows what they're doing. Although, even this part isn't straightforward. You can't assume that someone is good because their lashes "last". Some give 'false' lash retention by doing poor work (your eyelashes will appear to last longer when they're all stuck together! However, long term, you'll have damage to your natural eyelashes). The artist needs to be isolating every eyelash so that it can move and grow as it naturally would!
Secondly, you need to follow good aftercare advice, such as what is listed here. Many people falsely believe that they can NEVER get their lash extensions wet... GROSS! You absolutely must clean your lashes, unless you want a nice juicy eye infection perhaps? Cleaning them gets rid of all the normal dirt, debris and oils that naturally accumulate (your eyelashes exist by evolution to keep these things out of your eyes). Clean lashes means there is nothing sitting on there, weakening the adhesive bond. "But whenever I wash them, loads fall out!" I hear you cry! We naturally lose a few natural eyelashes EVERY DAY. If your lashes are clinging on via dirt, they will alllll come out at once when you finally wash them... as will any extra where the bond was weakened.
Finally. The natural eyelash growth cycle! As previously mentioned, we lose a few lashes every day, but everyones growth cycle is a little different. Lashes spend around 30-45 days actively growing (anagen phase), 2-3 weeks in transition where growth stops and the follicle shrinks (catagen phase), and then up to 90 days resting (telogen phase) until it falls out (ie "sheds") and gets replaced with new growth. In general though, most peoples full growth cycle in total is around 3 months or so. Now, we can't safely lash the newest growth, those little anagen phase baby lashes need to build a bit of length and strength before we go loading them with Extensions! So, if you do have extensions on, it's likely every extended lash will have shed (taking the extensions with them) within around 2 months or so. Don't let them come out this way though! Either get them infilled, or get them professionally removed. This is the best way to maintain the health of your natural eyelashes. If you continually let them grow out too far, this puts strain on the natural eyelash, and infills or removal will avoid this.
INFILLS: To maintain the look of eyelash extensions and keep on top of your natural growth cycle, you should be booking an infill appointment every 3 weeks. Some clients prefer to have infills every 2 weeks, and some can stretch as long as 4 weeks (in rare occasions, some clients can go 5 weeks!). At the Louska Beauty clinic, eyelash infills are done by time. So that, no matter how long you go between appointment, you will only ever pay for exactly how much work is done! Many other places simply price by the number of weeks since your last appointment instead, and/or by the type of set you had etc. There is no right or wrong way of pricing infills, we just find time the easiest and fairest way to do things, as it also rewards you lovely clients that follow the aftercare advice best as you won't need as long! What happens at an infill? - Your technician will remove any extensions that have grown out too long, possibly apply a primer or wipe over the lashes just to make sure they're ready to be adhered to. And then, using teeny tiny amounts of specialist semi-permanent adhesive, they will apply extensions to the new growth/naked lashes. To get the most out of these appointments, take care of your extensions and attend appointments with squeaky clean lashes! As long as you behave, and your lash artist knows their stuff, you can keep them on indefinitely. Good, properly sized and properly applied extensions, that are looked after well, will not cause any damage.
...So there you go! With Infills, eyelash extensions can last FOREVER!