5-7 mags of how many BB's? Are we talking mag ticklers? How are you shooting? Mostly in full auto or semi?
1400mah is going to get you anywhere between 1000-1400 rounds give or take. Typically 1 round per mAh but then take into account how inefficient a stock mech box/motor is, as it's probably not shimmed correctly and the motor height could be off.
But as stated, it's a crap battery and charger to start with.