Well i think it does depend very much on how often you use your file through a service. Also depends on how much your client suffers with lift and how much you have to file your product down and also what products you use.
I've found with Star nails files i get through them quite quickly, and as you say the more you use a file the less abrasive it is. I also use EZ Flow files and they seem to last longer and remain almost as abrasive as they were when new. I think the lower the grit the more hard wearing they are. My 240 grit files do seem to deteriorate quicker as a finer grit.
As long as you look after your files they will last a while or so, but it does depend on what make you use. Definate;y worth paying good money for files as they do last better.
