I had a quick look on the Walmart website but couldn't find anything, do you have the model number or product link??
If this helps I have the black version of this USB battery pack charge it's sold under a different brand in the UK - http://www.amazon.com/5000mAh-External-
It delivers 5.3v @ 1A max and claims to have a battery capacity of 5000mAH, needless to say it was capable of powering my PI with Wifi adapter.
As for how long it lasts while on full charge I cannot say I've never really tested it to find out.
I did do a recent experiment with 2 x AA 3000mAh Ni-MH batteries and a 5v step-up buck converter.
I got about 1 hours worth of X-Windows and streaming online mp3 music (Digitally Imported) via LAN before the PI died from lack of power.