No further melt can enter the cavity of the injection mold after the smallest diameter of the cold runner freezes off.
This duration is called freeze time and represents the theoretically maximum packing time.
While increasing the packing time stepwise it is possible to find out the freeze time by taking the weight of the parts. As soon as the weight stays the same, the cold runner is frozen.
In the simulation the freeze temperature in this area or the flow rate help additionally.