you can just add a lipo protect, here is one for 20 bucks http://www.kintecracing.com/Supervee_27_upgrades.html it that is too much there are some 4 dollar ones that buzz or something like that, but the one on that site has a soft cut off and works well with boats, i never had any problems with mine until i blew my esc and it was too close, it may even still work but my new esc's have lipo cut off.
one other option is to time your runs, start at 4 min and go from there