I found they crap out in about 6 mins, but then, so do the fingers.
I found the smaller ones, around 800 ma and smaller lose it even sooner as they don't have the mass to stay warm. I found the next size bigger prop helps, as the batt stays warmer. Bonus is a little better performance with the bigger prop, plus the denser air. If you can keep them warm in the plane by closing off the inlet and outlet ports you can get the full summer run time out of them. I use these little baggies on the profile planes made from the clear foam that electronics comes wrapped in. Tapes together with scotch tape and velcroed to the side of the plane with a velcroed closure. If the batt. comes out warm in your hand, your good to go.