A sound persists in our ear for 0.1 s. At temperature 22℃ in air and speed of sound 344 ms^-1, the total distance covered by sound from the point of generation and reflecting surface and back should be at least (344 m/s)x0.1 s = 34.4 m. Thus, the minimum distance between point of generation and reflecting surface should be half of it i.e. 17.2 m. So, in a long gallery and big halls, we can hear the echo due to the distance between the point of generation and reflecting surface more than the minimum distance required in given conditions.