From fit-tests run by Smart Air in India, we tested new N99 masks versus masks worn up to 30 days. Data shows that mask fit effectiveness was unchanged between the two masks, meaning fit does not get worse over time.
There is one caveat, however. Some masks have weak rubber band straps. These will stretch and get loose over time. That will make them fit less tightly. Masks with fabric straps (like the one we tested in India) retain their stretch better over time.