How long should fans run?
Dorn says, a rule of thumb is, the time (hours) to push a temperature front through a bin of grain is 15 divided by the airflow-cubic-feet per minute per bushel cfm/bu.
For example, a bin used for drying grain should be able to produce about 1.0 cfm/bu so it would take about 15 hours to push a temperature front through the grain (15/1 = 15). In another example, a bin equipped with a fan able to push only 0.3 cfm/bu could push a temperature front through in 50 hours (15/0.3= 50).
Corn being stored into the winter should be cooled as the outside air temperature cools, and should be dried to no more than 13% in an effort to retard any further growth of molds that may produce aflatoxin. Convection air currents are constantly moving in the bin, and depositing moisture in the top center of the grain where fines exist and molds will grow. Push warm and moist air out of the grain as the outside air temperature cools with aeration fans.
Source: FarmGate blog