If it's a company letter, it's up to the company how long they keep it. My guess would be it's permanent, employers need a paper trail to get rid of someone and would be loathe to ever give up that kind of leverage (just in case they ever need it).
As to how long it is subject to a PRIA inquiry, that should have a time limit for past employers, google the PRIA AC and read up on it.
But did you report the incident to airlines you've applied to?