Depends on your system and whether or not you're running your stereo.
I don't have the Epia, but I can get like 3-4 hours (?) running the comp, 15" monitor, 1 or 2 7" touchscreens, and stereo off my spare Optima, but I don't have to worry about cranking, since I have two batteries (soon to be 3). I think I get about 1.5-2 hours with the subs and the rear 6x9s turned on.
You could probably get at least an hour off a regular battery if you're not using the stereo.