Had a look at the battery current draw when one of the Poundland £1 1200 mAh USB power packs is not being used to power anything.
I had a pack where the cell inside had died and I had a spare cell, so before soldering it in I put a multimeter in the circuit.
It measured 1.5 mA from the cell to the DC/DC converter board, without any USB device connected. So that should give you an idea of how long one will last in standby mode.
