Roughly how long do you think I could run the empeg? And how long should I run the engine to recharge the battery?


There are just too many variables to give a definitive answer.

1) How many watts do your amplifier(s) draw?
2) How many amp-hour capacity is your battery?
3) How old and in how good condition is your battery?
4) How far do you want to discharge your battery? (deep-cycling a lead-acid battery is very hard on it)
5) How much current does your starter need to start your engine?
6) How many interior lights come on when you open your tailgate?
7) How fast will you run your engine during re-charging?
8) What is the output capacity of your alternator at that engine speed?

It's sort of like asking "How high is up?" About all I can suggest is you give it a dry run in your garage (with battery charger handy) -- see how long it takes before the battery is too discharged to restart the engine. Then you'll know. I can't think of any other practical method of finding out.

tanstaafl.