If eggshell would be reduced to ash, how many hours does it take to turn into quicklime?

eggshell is composed mostly of calcium carbonate, and if CaCo3 is burned at 850 degrees Celsius, it would turn to quicklime... so how many hours does it take to reduce it into ash and turn it into quicklime at 850 degrees celsius. PLEASE HELP!! desperately needed.. thanks!