NONE for a diode laser.
On my 10w machine, I set 254LPI and 90% power. Air assist made no difference. I kept reducing speed in 500mm/m increments, from 2500mm/m to 500mm/m. All it did was warm up the soapstone a little.
So I drop it in the 40w laser at 90% power and 1000mm/m to see if a visible light diode laser can do anything at all. If I tilted the soapstone in the light, I could see some marking, but it was not consistent. Oddly enough, the soapstone got barely warmer than with the 10w diode. This indicates the translucent rock was barely absorbing any power at all.
I saw black soapstone for counter tops, but it was too much $$ for me to experiment with.