I just ran a couple of quick checks with the USB meter I got today using one of my no name (Super Power Bank model FY-01) 12000 mAh and it has 2 ports 2.1 A and 1 A (about $8.50 on ebay). It read 4.97 V with the PMD661 sitting idle it drew about .36 A (I even formatted a card). I hooked it up to my SD MixPre-D with the cable made by DigiGal and it drew about .36 A. I hooked it up with my Josephson C42mp mics it drew about .52 A.
The battery should provide about 6.7 hours with digital in and 4.6 hours using my mics.