Speed of sound in air is 340m/s and the time our brain need to memorize between 2 sound is 0.1 second.
So, => speed x time = dist
=> 340m/s x 0.1s = 34m
Therefore,we need 34m to differently hear the echo and to recognize it by our brain, but the sound in echo is reflected back so
dist ( in echo ) = 2dist ( once it went to wall then reflected back to us) So minimum dist = d/2 = 34/2 = 17m