Each time interval between the original sound and the reflected sound must be the following to hear a distinct echo:

We will hear the same sound if we shout or clap near a suitable reflecting object, such as a tall building or a mountain, a short time later. The sound we hear is referred to as an echo.

The sensation of sound lasts about 0.1 s in our brain.

The time interval between the original sound and the reflected sound must be at least 0.1s to hear a distinct echo.