Question

What is the distance required to hear a echo?....explain briefly

Answer 1

velocity of sound in dry air is approx. 343 m/s at a temperature of 25 °C therefore the distance must be required to hear the echo is more than 17.2m 
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Answer 2

the echo is heard only if the distance between the person producing sound and the rigid obstacle is long enough to allow the reflected sound to reach the person at least 0.1 seconds after the original sound is heard. 

reason -
this because the sound persists in our ears for about 0.1 second after the exciting stimulus ceases to act .

now 
distance = (speed x time)/2
 
for air-
velocity of sound = 340 m/s
time = 0.1s
distance = (340 x 0.1)/2 = 17 m