You're a braver man than me! Yeh you could use the preverbial stop watch but then I guess speed, use and conditions will alter the speed your bettery drains as well. So be careful! You don't want any fires
(LiPo's actually scare me to death, so I'll stick with a built in cut off in an LRP or Orion etc just so I dont either burn my apartment block down, or myself haha)