What would be the estimated run time off of parallel 9V batteries fed through a LM7805 5V regulator (no peripherals)?
9V battery has about 500mAh, since your Pi consumes about 700mA, you're looking at run times about 40 minutes, more or less depending on the type of the battery (a good quality panasonic industrial type has 570 mAh.. low end type probably more like 400).
running Pi with 9V battery and a linear voltage regulator is very inefficient, you basically turn almost half of the energy the battery has into the heat. you should have considered running off 6V battery pack or with DC-DC converter.
Rather than a linear regulator, a switched mode regulator is much more efficient. Conveniently the MoPi board provides it and would accept 9V input with no additionally circuitry. A Model B with no peripherals and no load actually draws only 400--500 mA. A single alkaline 9V battery should provide enough charge for about an hour. A 9V lithium a bit more. Unfortunately, a rechargeable 9V battery is typically only around 300 mAh, that means less than an hour. Maybe just with a Model A. In comparison, the MoPi people show 8 AA Ni-Mh batteries (2600 mAh) to run a Pi for around 8-10 hours.