it really depends upon the motor, the prop, and the flying style that you do.
a 1806 motor on a 3S-850 can last for almost 20 mins with the correct prop (6x30) if flying reasonably. if going full throttle the whole time chop that in half, which is still a long time. most RC flights range from 3 - 6 mins unless flying a glider. if you get more than that you are doing great.
laters,
me