I was carefully filing one fret for ten minutes and it didn't do much, so at this rate it will take several hours.
Your first fret level and dress should take up to several hours IMO because there is a learn-as-you-go curve.
However, ten minutes is a long time for one fret, especially if you're not seeing a result. Can you explain your process? Maybe what tools you're using?

I could file a fret right down to the fretboard in that amount of time. (not recommended BTW, just sayin')