Don't just cool the surface - the interior must also cool. Provide uniform cooling on all sides to prevent stress. Completely cover slow cooling thick-walled parts with felt or blankets to block drafts. There's no rule of thumb for predicting sheet interior's cooling time. Factors include material thickness, ambient air temperature, and airflow to the part.
Experience is the best teacher.
As it cools, the sheet shrinks to reverse heat-caused expansion. Allow it to move freely to prevent stress. Shrinkage on the mold can also cause stress, it is best to remove the part as soon as it achieves dimensional stability. Be sure all heaters are functioning. Baffle heat on all sides.