With a Nitro plane, after the engine is broken in I fill the tank and fly it as I normally would for 8 minutes. My Spektrum radio has a countdown (or up) timer. I then bring it in and check out how much fuel I have left, usually about half a tank if I'm not WOT the whole flight. I then know I can push it out to roughly 14 minutes of average flying and still have a small reserve in case conditions are not good for landing and I have to go around a couple of times.

With electrics, same sort of system using a charger to check out how much a fully charged battery has drained. I fly them wherever I want for the first 8 minutes and then keep them close to me but at a decent altitude for the last few minutes, in case the battery hits reserve and loses motor power.