how long will it last?....well depends
OBK!
Different sized batteries/condition/amperage draw of components...well the list is long indeed.
Do recall from years ago was flying an S35 Bonanza at night, all the radios going (including DME, but no transponder in those days) and after an alternator 'failure' was still able to read the instruments after one hour, approximately.
Must have been a pretty good battery.
NB. The alternator 'failure' was due to the fact that the pilot forgot to switch on same.
When corrected, the ammeter pegged at 100 amps.
Big red lights....fortunately no failures, all instruments were illuminated red....ah, the good 'ole days.