Every client has different experiences with how long their eyelash extensions last. Around week two, you will notice your eyelash extensions will begin to fall out, sometimes with your natural lash, sometimes without. Don't freak out, this is normal! Eyelash extensions have a growth cycle and when their growth cycle is complete, they shed so a new lash can come in. If you have a black extension on your natural lash while it sheds, of course you will notice it. But again, this is very natural and it does not mean that you're extensions are causing any harm to your natural eyelashes.
Around 2-3 weeks is when I always tell clients to come back in. After four weeks, you will basically need a new set of eyelash extensions. It will always be cheaper to come in sooner rather than later. Not to mention, the sooner you come in, the more full those lashes would look! And who wouldn't want that!