want to be sure???????
charge battery fully
disconnect negative cable
get accurate voltmeter and install in series between neg. battery post and neg. cable. set meter to DC AMPS
turn on parking lights
read meter
take meter reading and divide that # into the "amp hours capacity" listed on your battery. that # is approx. hours you can expect a fully charged good battery to live.
that will give you a ball-park idea what to expect.