Very rarely do I ever run into a bad file.
One thing that will kill a chainsaw file is work hardened saw teeth
where you hit something harder than wood and the impacts hardened the
steel.
The file will skate over the hard surface and not take a bite.
You should be sharpening when four strokes per tooth will finish the
job.

Four good sharpenings is not bad you might be able to get six if you
keep the chain out of the dirt and only file on the push stroke.
If you want eight sharpenings per file get in the habit of wiping the
filings out of the file on your pants leg after each tooth.

A few dozen? maybe in the movies.