The rof of the UBT is given as 800-1050 rounds per minute. So at an assume 900 rpm, the ammo would last for
top: 12.5 seconds
middle 16.5 seconds
bottom 23 seconds
in comparison, the (in my opinion) dreadfully short clip of the Lancasters .50 tailguns has a firing time of 25 seconds, the Ki-67 rear 12.7mm have 27 seconds (400 rpg) and the top 20mm (250 rpg) has 21 seconds of cannon fire.
The current incarnation of the B-26 has a rear turret firing time of just over 1 full minute...