How long will my x 2 23a 12v battery last running a 300mah camera?
Assuming two A23 batteries in parallel at \$12 V\$, the charge available is about \$110mAh\$ ( \$55 mAh\$ each). So, converting the \$12 V\$ to \$5 V\$ with average efficiency, the capacity will be equivalent to \$220 mAh\$ at \$5 V\$. So, it would last about 220/330 hours or about 40 minutes.  

Ref:https://en.m.wikipedia.org/wiki/A23_battery.