I am going to assume that you are referring to a solid. For liquids and materials going through a solidification, this would be different.

You need to know the initial temperature (900 ºC ?), the final temperature (300 ºC ?), the physical dimensions of the work piece, the thermal conductivity of the material and the ambient temperature.

Suitable finite element simulation software such as ANSYS would help.

Quenching word is sometimes used to cool a liquid fast enough to form a glass. If this is a case and you want to quench from 900 K to 300 K, the rate of quenching will depend upon the way you are quenching. You may simply take out your sample from furnace and blow air on it. You may drop it in ice cooled water or in liquid nitrogen. In all these cases quench rate will vary and the temperature diffence between hot state and the temperature of the quenching medium will be important. Cooling rate will vary as the temperature difference will decrease. Only an average cooling rate can be determined experimentally. The exact  theoretical calculations will be difficult as the viscosity change of the medium from liquid to solid will be very fast. This will depend from material to material.

While quenching, probably, you would prefer material characteristic. not to get change drastically. You can take, as rule of thumb, about 4 degree per minute reduction