You can also often set the ESC to "soft cut" the power, which I prefer.

Couple this with a higher cut voltage too, to protect LiPo's if you use them...

What happens is that you will suddenly notice a marked decrease in power available on the plane.

The prop will still spin and you will have enough power to go around and land, but you'll notice that the plane will not want to climb as before no matter what you do.

This indicates that it is time to bring it down ASAP. 2 minutes later you end up at full cut off.

Normally way before I hit this point, the gradual voltage drop from the batteries becomes very apparent.

I find that ( depending upon the plane ) I can tell that soft cut is 2 to 4 minutes away...