Most car batteries are roughly 50-70Ah. Which means about 720W for an hour. Divide 720 by current drain and there you have it. Look in amp/head unit manual for max current draw - or double output of amp (ass. 50% eff). Or add up fuses - my headunit 10A (must take less!) and amp 40A max poss 50A - therefore I would get twitchy after 1 hour at reasonable volumes, but you might get 2 at low volume. I had a friend back in school days whose PUG 309 battery flattened in 15mins with his system on!
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