Question

what factors are important to get distant echo

Answer 1

in order to be able to hear the original sound and it's reflection distinctly, at 22oC velocity of sound in air is 344m/s.

Answer 2

The factors are important to get distant echo are-
1) Size of wall should be greater than amplitude of sound waves. 
2) 17.2 m should be distance from wall from which sound is getting reflected.