Per filing? I used to use about 2 old Oregon files to sharpen a 32" chain that hit pavement without bearing down into it.
I've gotten my files by the dozen from Frawley's or Stahl's, so you could try ordering from them. Could be that I run my files duller than most, but that wouldn't explain my poor luck with other brands.
Another thought that just came to me regarding how I file in a vise. Once the initial sharpness is gone on a file, and I start bearing down on it some, that could make for ideal cutting conditions. Many cutting tools have an ideal feed rate: too fast, and the cutter can wear fast or get damaged, too slow and it dulls fast by abrasion rather than cutting. This is most important when it comes to machining metal on a lathe or mill, it could be a similar case with the file that I'm able to extend it's life by bearing down on it harder.