Mark,
I have run a variety of different batteries, on different motors, and can suggest that at KMRC, a 5S 5500mAh is enough for about 21 minutes. This is on both 1700kV Tekin and a 1700kV Losi motor. If I were to use a 1400kV motor, and gear up a bit more (to acheive a slightly higher top speed than before), it is likely that I'll get a little more run time, with less tyre shredding acceleration out of the corners. As I'll use full throttle less, I'll conserve capacity. To get any run-time out of 1/8 electric, it is vital to be smooth on the throttle, it'll also make the car more driveable, as they are far more aggressive than nitro.
Obviously, you are probably a little smoother on the throttle, so could probably get away with a 5000 or 4500mAh pack.
I have actually decided to go the other way with my stuff for next season- have a very light car, with enough capacity for 10 minutes, then pit for a new battery. I just need to work on a fairly fast way of changing batteries. (I'm thinking velcro on body, no clips, a quick-release battery strap, and polarity-protected connectors, like EC-5s).
The total stop time shouldn't be more than two nitro pit stops (pretty much all nitros do two pit-stops in 20 minutes), and then, when electric takes off enough to have our own seperate class, we'll run 10 minute finals, and you won't have to buy new batteries to suit.
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