How long a battery will last depends on the power being used, which is a function of voltage and resistance. You cannot judge how a long a battery will last just based on voltage or amperage (current). The battery is rated in amps or milliamperes and how long it will last is the amperage being drawn by the device. Voltage is the pressure pushing the current though the battery (like water in a hose) and the amperage is the size (diameter) of the hose. The more pressure and wider hose allow for more amperage (water volume) to be delivered, otherwise known watts (power). You have to remember that most of these devices are used in boost mode, meaning the battery is always used at 3.2 to 4.2 (starts off high after a full charge and goes down as its stores are depleted) volts and the circuitry is upping its voltage to over 6 volts for the PWM circuitry to operate. So to measure amperage being used you must know the voltage of the battery and how many amps are being drawn. That will give you the current draw (power). The current being used is what will determine how fast your battery will drain. Also, remember if there is any heat in the battery tube that is wasted energy as the process inside the battery converting its stores to usable electricity is not 100% efficient.
