You don't want to discharge lipo below 3v/cell or 6v for a 2S lipo. You can damage one that way. There's a Novak 2S Smart Stop that's set for 6.25v like this -

http://www3.towerhobbies.com/cgi-bin...?&I=LXPGM8&P=7


Or you can try timing your runs which if you're bashing usually doesn't work well. On a track, timing runs works pretty well.

If you assume about 200mah/min, then you'd have 20min run time. Thats prob on the pessimistic (low) side. You don't want to pull more than 80% of a lipos capacity during discharge - you won't get good life that way.