With just the radio on with no additional amps or lights you should be able to last a few hours at least. Keep in mind that draining and recharging to that extent (close to dead) will kill your battery for good pretty shortly.
Darkness is deaths ignorance, and the Devils time.
'96 Grand Cherokee Laredo
'93 Taurus SHO
'77 Corvette L-48
'74 Land Cruiser FJ40