Flash memory has a "write life". Without getting into the physics behind it, that means that you can only write data to the "cells" (the individual memory locations) so many times before the cells stop responding correctly. Reading data from the flash memory has almost no effect; you can often read data perfectly from flash memory that can't be written to anymore.
It's not writing more data to the key that does the damage, it's writing to the same cells. Like a hard drive, when you delete a file (normally), the file isn't really deleted, it's just "forgotten" (the index entry telling the system where to find the file is changed). Until the drive starts to get pretty full, new data is written to "fresh" locations. It's only when there isn't enough "fresh" space that the old space starts to get re-used.
So the overall picture is this: if you keep the USB key nearly full and change the files a lot, you will wear out the cells quicker. If the USB key is only ever partially full, or if you mostly read files from the key and rarely write to it, it should last quite a while. If you write once, then do nothing but read after that, the key will (theoretically) last until it's physically damaged. (That said, you should always have at least two copies of important data in two different places.)
You should note that the number of write cycles that flash can undergo has increased dramatically over the past few years, to the point that it is practical now to use flash-based solid-state disk drives (SSDs), even in high-activity data center servers. That doesn't necessarily mean that your $9.99 4GB USB key is going to be that reliable--there's a reason why a 128GB SSD costs more than 128GB worth of USB sticks--but it will probably be a lot better than the $150 256MB USB key that you could buy six or seven years ago.
, but im not sure :/ 