What is the minimum distance of echo?

Depends on the quality of the sound. The minimum distance to hear an echo will be partly based upon the observers ability to resolve different sounds (i.e. so that they can distinguish between the actual noise, and the arrival of the reflected sound - a little 100 year old lady may not be as good as a 15year old!). This will vary between observers, as their auditory reaction times vary

In the perfect Physics world though:

Assuming a sharp sound (like a gunshot) the minimum distance (d=vt) to hear distinct echo is 17.2m considering the speed of sound to be 344ms.

In general the "minimum echo distance" is always equal to half a wavelength from the reflector.