Mine seems to vary, last time I tested I think it was 10ppm, tryed a few months ago and it was 5ppm. Now my test kits out of date .
My "overstocked" tank is anywhere between 0ppm and 20ppm right now (kits just out of date so cant test right now, but I've never seen it above 20ppm, and it's usually at 0ppm) .