Your VERTIMILL grinding media should be recharged whenever your VERTIMILL drive motor power draw drops below the minimum set point. More frequent charging, adding smaller amounts, will result in steadier power draw and steadier performance.
Modern VERTIMILLS used in lime slaking duty are designed to operate best when this power draw is between 70% and 80% of of full load amperage (FLA).
Never exceed 80% of FLA on the Vertimill drive motor.
Power draw dropping can also be an indicator of screw liner wear.