Not sure what I am missing. Your first post looked like you had goood flight times, the second that you were dissapointed.
I doubt that a 7.4V will burn out the motor. I have burned up a few with 11.1V on my 3DX though.
I would imagine that the 2400 should give you plenty of flight time.
If my math is right 2400 mAh = 2.4 Amps for one hour, or 2.4 Amps for 60 minutes
I would guess that 3/4 throttle would use about 6 Amps with a 1260.
You should get 2.4/6*60 minutes ie. 24 minutes .
Mmmm something is wrong. Where is DR Kiwi when you need him?