The cooling capacity is not a function of the volume of coolant, but of the heat dissipation of the rad.
If you want to rely on the volume of coolant, then you need to be aware that the coolant temperature will steadily increase, until it reaches a balance point, where the rad can dissipate all the heat. (because the rad is more efficient at a higher temperature).
If you want to run the numbers, then you're talking about Watts (and watts per hour) and BTUs, and the heat capacity of water (or coolant).
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