The 14,000 mA.hr rating (if it's real!) would probably be measured at 5V (USB voltage) and relatively low average current, as used by the main devices that it is intended to power (mobile phones and tablets etc). While these things can pump out high current for short bursts, they can't always maintain high sustained current, due to problems with overheating etc. Also, their effective A.hr rating falls off at higher power demands (i.e. higher currents and higher voltages).
Remember that the total energy stored in a battery is approximately equal to the voltage times the average current (amps) times the duration.
Ignoring the reduced efficiency at high voltage / current, 14 A.hr @ 5 V is approximately equal to 70 W.hr - that's about right for a 450 g LiPo battery.
http://en.wikipedia.org/wiki/Lithium_polymer_battery
However, when running at 12 V output, the best you could do is about 5.5 A.hr, which MIGHT give you three hours or so if you're lucky (assuming the thing can maintain a sustained output of 1 to 2 A @ 12 V without overheating).
The other thing to remember is how long will it take to recharge a 14,000 mA.hr battery via a USB cable? If you're lucky, you have a 2 A USB charger handy, so you can recharge it in 7 - 8 hours, but if you're trying to charge it off a typical 1 A phone charger or 500 mA computer USB port, it will take MUCH longer to recharge.
You'd be much better off with something like a 12 V Sealed Lead-Acid Battery (SLA). You can buy these (and suitable chargers) from places like JayCar - they're a lot cheaper, and will give you a genuine output of up to 20 A.hr or more at 12 volts and sustained currents of 1 - 2 A or more.
and you'd need to add a decent charger.
