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 between the point of generation and reflecting surface should be at least (344 m/s)x0.1 s = 34.4 m. Thus, the minimum distance between the point of generation and reflecting surface should be half of it i.e. 17.2 m. So, we cannot hear an echo in a small room as the minimum distance for the echo will not be satisfied. But we can hear an echo in a long gallery and big halls.