Question

When can we distinctly hear the echo of a sharp sound

Answer 1

An echo can be heard only after 0.1 sec from the original sound and there should be a distance 34.4m.

Answer 2

An echo is heard distinctly when the minimum distance between the source and observer is 17m and the sound persists for 0.1 seconds in the ear.
The intensity of sound produced must be enough so that it can be heard after refraction. 
The size of reflecting surface must be larger than the wavelength of sound waves.