Originally Posted by CruisinGA
Best thing to to in this case is empirically determine.
second this... You'll get a real answer and there's no harm in running down your battery. The JL Amps shut off automatically at low battery voltage, plus the Optimas recover just fine from deep discharge.
As far as the exponential decay, yes there's a formula that uses Peukert's number (basically a battery efficiency constant) for calculating run time, but overall lead acids have a linear discharge profile. Don't really think that would be useful though, because there's no way you'll be able to figure out the current draw on the battery because your stereo is a dynamic load.
Also intermittent loading of a lead acid battery helps capacity allowing the chemical process to recover so your stereo probably isn't that hard on your battery, wouldn't hurt to start your boat up every once in a while either.