The assumption above is that you are talking about receiver NiCD / NiMH battery packs, not actual flight power batteries.

With NiCD / NiMH packs you can loosely evaluate remaining flight time by the voltage level when the battery is under load.

With LiPo ( and other ) flight batteries, the voltage level stays relatively constant until the end of the battery capacity at which point it drops off VERY quickly... ( some may say almost instantly! ).