I feel kinda weird: I've cut though a lot of fabric, but I'm only on my second blade. How odd.
Cathy's husband's got it right. You just sharpen until it's sharp. I've sharpened knives, chisels, and needles. Some took a few minutes, some took over an hour (that was a v gouge chisel including the regrinding time.) If you run a blade in a sharpener and it's less dull but not sharp simply keep going. I'd like to try a sharpener but haven't needed to sharpen my rotary cutter blades yet.