Well slug that all depends on the output graph of the alternator. Some alternators start making 80-100 amps @ 14.4V at about 1800 RPM though most start at around 2200rpm making serious current.
I guess it depends on how long he is "idling" the car for and how much heat is being generated and how quickly his start and auxillary battery take charge.
A place I'd rather be...
Mitchel Road, Bomjinna. Central West Victoria (Jan '08)