Some back of the envelope (well, iPhone calculator) maths, if you left a PAW powered by the battery with everything else off...
Whilst a PAW can draw up to 2.1A, generally when running it draws an average of 750ma, or 0.75A. But that is at 5v, so we need it convert it to 12v, by going via Watts.
Watts = Volts * Amps
At 5v this is 3.75W.
So now back to Amps for 12v, but we will add 10% to the Watts to account for the converter being only 90% efficient.
Amps = (Watts * 1.1) / Volts
At 12v that is 0.34A
Them for an approximate discharge time, divide the battery Ah capacity by the draw in Amps at 12v to see how long it will last. Though to be able to start assume it needs 25% charge remaining.
So for a 20Ah battery, that equates running it flat in ~59 hours. For engine start then ~43 hours.
Naturally, finger in the air roughly, your milage may vary, objects in the mirror are closer than they appear, and such disclaimers apply.