With no screen or audio, you just need to know how many amps your board takes. I have heard around 1.1 amp at 12v. Let's say it is 1.1 amp at 12v, so with 7200mAh, you would divide the GC amp requirements from your battery:
-Find amp draw at 7.4v by converting to watts
1.1A * 12V = 13.2W (Amps * Volts = Watts)
13.2W/7.4v = 1.78A
-Divide battery Ah by amp draw
7200mAh = 7.2Ah
7.2Ah/1.78Ah = 4.04h
So around 4 hours, if any that's accurate.
If I were you, I would find the amp draw of your board at that voltage and then use the same calculation as above. At lower voltage, amp draw will be higher.
