Does anyone know how long the rings that do the sharpening last? The one I have from JOann's, the hand held one, has what seems to be a sand paper-ish kind of ring on one side and a slick/smooth ring on the other. You do the sandpaper side first then the smooth side...but how long do those last? I have never seen anything or read anything about replacing them, and common sense says they won't last forever... any thoughts?