Question

what is the standard unit of Echo

Answer 1

echo in the continuity of sound due to reflection of sound
and for echo there is no specific SI unit
echo is sound and hence its SI unit decibel

Answer 2

An Echo (also known as Delay) unit creates a copy of the original signal and after an interval (usually selectable within a limited range) replays it one or more times. It simulates the way a sound can be reflected by a single acoustically reflective surface (a cliff, for example).