3rd battery?

I am wondering if I need a 3rd battery for my stereo? or if my deep cycle battery will do? I already have the battery isolator where it charges when the boat is running and I usually dont sit for more than a few hours without starting the boat? that said I have a hifonics zxi8008 740 watts and am going to add a hifonics zxi 8408 880 watt amp. is there a way to figure out about how long the battery will last? without starting the boat? any info or suggestions would be great thanks