You can contact the furnace manufacturer for a proper procedure. The normal practice is to burn the carbon at 1500 deg. F for 4 to 5 hours, once a week. It really depends on how much carbon build up you see in the furnace. The Surface Combustion procedure says for a proper burn-out, the gas analysis in the furnace should be:
0.5% - CO2
15.0% - O2
0.0% - CO
You need to control the amount of air that you put in as overheating from high concentrations of carbon on the alloy could cause damage.
I hope this helps.
Rao Yallapragada