Sometimes, I will keep using a file, as I have the time to put up with it not cutting as well as a new fresh file, other times, I get a new one and use it as time is limited.
Old dull file on a chain that needs lots of material taken away = misery.
Old files are still ok to use on wood, or other projects, or doing a prelim swipe on an unknown chain where it may have hardened teeth from being overheated on a grinding wheel, saves a new file becoming a useless sq one in one pass.
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Old files are ok for teaching someone how to sharpen well, as little is taken off on each pass, and if their elbow/ arm is waving about, not as much material is taken off when learning, most have only used dull dull dull files anyway, and find what I consider dull to be near light saber status.
Some chains are harder than others, and what might feel like its dull on one chain, cuts well on a softer chain etc.
I like to get my moneys worth out of them, but as mentioned, they are a consumable item, so buy cheap, and stack it deep.
Ive used cheapies, some worked well, some didnt, used stihl, husky, oregon, pferd, vallorbe, and they all worked well, settled on what ever was best value for money at the time by the dozen, so have a few of each in different sizes (thats another world of pain, so label the gear clearly).