Based on SD table, a 722 consumes about 20 watts during record, with phantom power on, recording to the HDD. Your KM140s draw about 2 ma each which is lower than many condenser mics, so 20 watts is a reasonable number. You batteries supply 7.4v * 6.9 Ahr = 51 watt hours. 51 Whr / 20 W = about 2.5 hours. In my experience, 2.5 hr is a low estimate for a 6900 mAhr battery, but it depends on the particular battery.
However, there is no reason to guess or estimate battery life since it is an easy test to find out the life of a particular battery. Just set up you 722 with mics attached and phantom power on, connect a fully charged battery, and hit record. Obviously, the 722 will stop recording when the battery is dead (the 7xx recorders are good, but they still require power). Then check the length of the recording. You may end up with more than one file depending on how long the battery lasts since FAT32 has a max file size of 4 gig. You probably should set the file size to 4 gig (menu item 35) and you may want to record at a lower sample rate and at 16 bits to keep from generating multiple files during the test. Or record a FLAC or MP3 file to keep the size down. You get the idea. After the test, just write the time on the battery. Don't do this test too often since Li-ion batteries do NOT like to be deeply discharged.
In practice I never approach the endurance of my batteries since I NEVER run the 7xx boxes with just one battery; I always use an external power source along with the internal battery. Dual power sources is one of the great features of the 7xx boxes. I used to connect to AC if power was available, but now I use an external battery.
I have two types of external batteries. The first is two 7.4 V Sony mount batteries in series which is 14.8 volts. I built a battery sled that holds the two batteries. The advantage of this is it uses the same batteries that mount on the recorder.
The second battery I use is this:
http://www.batteryspace.com/waterbottlelifepo4battery128v6800mah8704whforhidhalogenbikelighttrial-techfemaleplug.aspxSince I am a bicyclist also, I actually bought this battery as part of this bicycle light kit:
http://www.batteryspace.com/trailtechsinglehidhelmetlightwith128v64ahlifepo4waterbottlepacksmartcharger--2000cyclelifeforbattery.aspx(This is a really nice bicycle light, in case you care about such things.)
Anyway, that bad boy battery will run a 7xx box for 8 hours easy, and will charge the internal battery as well.
The biggest problem with using an external battery is building the cable. Soldering to the Hirose 4-pin connecter that SD uses is not an easy task, particularly for these old eyes.
Based on email correspondence with a SD engineer, do not exceed 18 V with an external power source. I mention this because many nominal 18 V batteries can have a much higher voltage when fully charged.
One more comment: I always record to both the HDD and CF. The HDD does not draw that much more power and being able to record to dual media is a great feature. A long time ago I was using a 702 without an external drive. The 702 recorded just fine, but when I transfered the recording from CF to my laptop, Windows f-uped the CF. I had to use a hex editor to extract the data, and I delivered the recording about a week late. I HATE going to a client to try to explain "technical difficulties". At least I was able to recover the recording. So the moral is: always record to dual media.
That's my experience.