With my car RUNNING and my Yellow Top battery I can only run my lights and system for 15 minutes TOPS before I drop below 10 Volts and the inverter cuts out.
Honestly, the length of time you can run your system on the battery alone very much depends on how hot a system you have (how much draw it uses) and how LOUD you crank it.
Greg sapped his yellow top last year in a matter of minutes down in KC by making his system thump a bit. That was a good time too.
My opinion? Invest the money in a jump pack and keep it with you all the time. It's good insurance.
-Alex