How much is too much?  For a  large heater, one that heats an entire apartment or 3 or more rooms in a house, no more that about 45 kilos of wood should be burned in a 24 hour period.  This amount of wood is usually burned in two cycles, one in the morning, one at night, somehwere near 8 hours apart.

The firebox should be filled with a load of wood and then set a fire and allowed to burn to coals without unneeded disturbance or opening of the heater door.  Any time the heater door is opened, the firebox temperature drops and secondary combustion (the very effecient burning of wood) can be undesireably stopped.  If once the first load of wood is burned and a second is added, the bricks in the heater will start to absorb more heat than they can radiate (give off).

If  bricks become supersaturated with heat they may warp or crack, which is obviously disadvantageous for the longevity of the heater.