Google is good for this. N95 and higher is effective. Mask application matters but they're effective without 100% seal. I don't think they get less effective with time, just harder to breathe through.
They do get less effective with time, exposure to moisture. Using alcohol to clean an N95 will significantly reduce its effectiveness.
Early on, Stanford did a study on various methods of sterilizing N95s and the change of effectiveness. Heat (70°C for 30 mins) was best.
https://news.stanford.edu/2020/04/01/researchers-show-how-to...